It's kind of funny, but I have a friend who wears two pairs of underwear.
You might think it's strange, but one of my friends likes to double up his underwear.
この場合、次のように言えますよ。
ーIt's kind of funny, but I have a friend who wears two pairs of underwear.
「ちょっとおかしいんだけど、俺の友達はパンツを２枚履くんだ」
to wear two pairs of underwear で「パンツを２枚履く」
ーYou might think it's strange, but one of my friends likes to double up his underwear.
「おかしいと思うかもしれないけど、俺の友達はパンツを二重にするのが好きなんだ」
to double up one's underwear で「パンツを二重にする・二枚重ねにする」
パンツの数え方は次のように言えますよ。
a pair of underwear「パンツ一枚」
two pairs of underwear「パンツ二枚」
three pairs of underwear 「パンツ三枚」
ご参考まで！
カナダ 
