You can combine both questions into one sentence by asking:
Who did you eat dinner with and where?
It is easier to start with the 'who' because it is a longer sentence than asking 'where'.
I hope that helps!
２つの質問を１つの文にまとめることができます。
Who did you eat dinner with and where?
who（だれ）のほうが where（どこ）の文より長いので、 who から質問を始めると簡単です。
参考になればうれしいです！
こんにちは。
Where and with who did you eat dinner?
どこで誰と夕飯食べたの？
上記のように言えます。
What, who, when, where, why を同じ文で and を使ってつなげて言うことができます。how もいけます。
例：
When and where is the party?
パーティーはいつどこでやるの？
What were you doing out so late and with who?
こんなに夜遅くまで誰と何してたの？
ぜひ参考にしてください。
You have used the pronoun 'whom' instead of 'who' in this case and it is perfect.
For example, you may say: 'one person in whom I had taken an interest is John'.
You may also say: 'I would like to know whom you told about this story'.
You may also ask: 'Whom did you go to the party with?
'Whom' is used in questions when you ask about the name or identity of a person or group of people.
As for the pronoun 'who', it is appropriate for questions like: Who is it?
Who did this? Who cooked this food?
You may also say: 'What is happening does not define who we are'.
この場合は'who'ではなく'whom'という代名詞を使っていて、それがバッチリです。
例えば、'One person in whom I had taken an interest is John.
（私が興味を持った人の１人はジョンです。）'ということができます。
また、'I would like to know whom you told about this story.
（この話についてあなたが誰に話したかを知りたいです。
'ということもできます。
さらに、'Whom did you go to the party with?
（だれとそのパーティーに行きますか？）'という質問もできます。
'whom'は、人やグループの名前や招待について尋ねる質問の中で使われます。
代名詞の'who'は、以下のような質問のときに適切です。
'Who is it?
（それはだれですか？）
''Who cooked this food?
（だれがこの料理を作りましたか？）
'また、'What is happening does not define who we are.
（起こっていることが我々が誰であるかを定義するわけではない。）'
ということもできます。
Please tell me everything about your evening meal out!
Please share all the details about your dinner!
I suppose you had dinner with a close friend at an expensive restaurant?
instead of asking the question directly, it may be more fun to ask a very open question about the evening:
"Please tell me everything about your evening meal out!" The answer to this question should include 'where' and 'with whom.'
Similarly:"Please share all the details about your dinner!"
A rather different way to find out about your friend's meal, is to suggest:
"I suppose you had dinner with a close friend at an expensive restaurant?" If you are mistaken, your friend will be able to correct you - and so give you the details you would liken to know.
率直に質問する代わりに、もっと楽しく尋ねることが出来ます。
例
"Please tell me everything about your evening meal out!"
（夕飯について全部話してみてよ）
この質問に対する答えは 'where（いつ）' と 'with whom（誰と）'が含まれます。
同様に
"Please share all the details about your dinner!"
（あなたの夕飯について詳細を教えてよ）
友人の夕飯について確かめるのが、少し困難な場合は次の表現をお勧めします。
"I suppose you had dinner with a close friend at an expensive restaurant?"
（誰か親しい友人と高級なレストランへ行ったんでしょ？）
もしあなたが勘違いしていたら、お友達があなたの知りたい詳細を正しく教えてくれるでしょう。
Where did you have dinner today and were you on your own?
Did you have dinner with your friends at that restaurant today?
Where did you guys go for dinner?
MEANING= this is an informal way of asking where did you go for dinner, but because I said ‘guys’ implies that you were with people, so if they were with people their reply would be ‘yeh me and ____ went to this restaurant.
Where did you have dinner today and were you on your own?
MEANING= this is a bit more formal and I think is a little bit like an interrogation.
This is a confrontational way to ask where and who did you eat dinner with?
If I was asking my friend this question I would split it into 2 questions.
I think it is friendlier that way.
Where did you go for dinner?
意味：これは、相手が夕飯にどこへ行ったかを聞くカジュアルな言い方ですが、guys という言葉を使っていますので、他の人と一緒だったことを示唆しています。
なので、もし相手が他の人と一緒だったら、'Yeh me and ___ went to this restaurant.（うん、私と___ はこのレストランに行ったよ。）'のように返答があるでしょう。
Where did you have dinner today and were you on your own?
意味：これはよりフォーマルな言い方ですが、私は少し尋問のような感じがすると思います。
だれとどこで夕食を食べていたのかを尋ねる会話的なフレーズです。
もし私が友人にこの質問をするなら、２つの質問にわけると思います。
そのほうがフレンドリーな聞き方だと思います。
>Where did you eat dinner? Who joined you?
*This is two sentences. I like the idea of two different questions, makes it easier to answer.
.....................
>Where did you go for dinner and did you go alone?
*By asking if he/she went alone is asking indirectly if someone joined him/her for dinner.
.................***.............
Where did you eat dinner? Who joined you?
個人的に、2つ質問をすることで、回答しやすくなるので気に入っています。
Where did you go for dinner and did you go alone?
一人で夕食にいったか、誰かと夕食に行ったかを尋ねる表現です。
Where did you go for dinner and we're you alone or with company?
You can make these two questions into one by using the conjunction "and" to connect the different phrases or clauses. Each of these two questions can be used without asking more than one question. Here they are:-
1. Where did you go to dinner and with whom?
2. Where did you go to dinner and we're you alone or with company?
これは接続詞の "and" を使うことで、一つの文で表すことができます。
以下の例文はどちらも、この二つの質問を一つの文で尋ねています。
1. Where did you go to dinner and with whom?（夕食はどこで誰と食べたの？）
2. Where did you go to dinner and we're you alone or with company?（夕食はどこで食べたの？一人で食べたのそれとも誰かと一緒？）
Use the sentences above to ask how dinner was and who they were with.
The last sentence doesn't ask how enjoyable dinner was but it assumes that it was enjoyable, if the person didn't enjoy they will respond saying so.
These setences contain two quesrtions in one sentence.
This is possible in the English language so long as it is constructed correctly, we use conjunctions (joining words) to join two sentences or two points.
Use the acronym 'fanboys' to remember your joining words:
F = for
A = and
N = nor
B = but
O = or
Y = yet
S = so
"I like to eat chocolate for it tastes great!"
"I have some apples and I'm going to eat them all!"
"I don't like eating soup nor do I enjoy peanut butter."
"I am leaving for work but I would rather stay home."
"Do you like to swim in the ocean or do you prefer the swimming pool?"
"He has a toothache yet he still eats candy."
"I am tired so I am going to bed early tonight."
夕食を誰と食べたのかと、その感想を聞きたいなら、上記の文が使えます。
最後の文では食事の感想を聞いていませんが、「誰と楽しんだのか」という聞き方をしているので、相手が否定しない限りは「楽しんだ」と解釈していいと思います。
これらの文では、二つの質問を一つの文で尋ねています。英語では構文さえ正しければこのような聞き方が可能です。二つの文をつなぐとき、英語では接続詞を使います。
接続詞は、'fanboys' という頭文字で覚えましょう。
F = for
A = and
N = nor
B = but
O = or
Y = yet
S = so
"I like to eat chocolate for it tastes great!"（チョコレートはおいしいので大好きです）
"I have some apples and I'm going to eat them all!"（リンゴが何個かあるので、それを全部食べます）
"I don't like eating soup nor do I enjoy peanut butter."（スープもピーナツバターも好きではありません）
"I am leaving for work but I would rather stay home."（仕事に行かないといけないですが、できれば家にいたいです）
"Do you like to swim in the ocean or do you prefer the swimming pool?"（海で泳ぐのが好きですか、それともプールの方が好きですか）
"He has a toothache yet he still eats candy."（彼は歯が痛いと言っていますがそれでもお菓子を食べています）
"I am tired so I am going to bed early tonight."（疲れたので今夜は早く寝ます）
おっしゃられている内容は、以下のようにも表現できると思いました(*^_^*)
Who did you have dinner with and where?
盲点をつくすごく良い質問だと思います(*^_^*)
ちなみに、2つ以上の疑問詞がある場合の文の作り方については私のブログでたくさん紹介していますので、興味があれば見て下さいm(__)m
英語でどう言う？「誰が何て言ってたの？」(第6回)
英語でどう言う？「何時から何時まで仕事なの？」(第10回)
などを検索すれば出てきます(*^_^*)
「何がどこにあるんだ？」とか
「誰が何をしたの？」などのマニアックな言い方を知ることが出来ます(*^_^*)
以上です。
少しでも参考になれば幸いです(*^_^*)
Who did you go to dinner with and where did you go?
Did you go to dinner by yourself or did someone accompany you?
Which restaurant did you go to for dinner and who attended?
You can use all of the above to ask the questions, whether you choose one sentence or two sentences is fine."Where did you go and with who did you go?"or "Where did you go for dinner?" "Who did you go to dinner with?"
Accompany: If you accompany someone, you go somewhere with them.
上記の文どれも使えます。
これは一つの文で言うことも二つの文で言うこともできます。
"Where did you go and with who did you go?"「どこに誰と行ったの」
または
"Where did you go for dinner?" "Who did you go to dinner with?"「夕食はどこに食べに行ったの」「夕食を誰と食べに行ったの」
Two examples. The first one consists of the question with whom and where. The second one is two separated, so it depends on how you want to express yourself.
イギリス 
南アフリカ 
アメリカ合衆国 
カナダ 
日本 
オーストラリア 
