10$札と20L$札半分ずつくださいって英語でなんて言うの？

You would like Yens exchanged into dollars and require an equal number of tens and twenties. Well, it depends whether you want an 'equal value' of notes or an 'equal number ' of notes. The first example sentence above is a little unclear. The second example is very specific and is a value split. The third example is a number split. So to be sure, you need to be very specific!

"Can I exchange my money into 10's and 20's?" Here is an easy way of asking if you can exchange (swap, or trade something in for something else) into 10's and 20's.
"Can I please get half in 10's and the other half in 20's, please?" This is a polite way of asking them to change half of your currency into 10's and the other half into 20's.

"May I have one half in 10's and the other half in 20's" A very polite and formal way of asking, 'May I' can be used in many contexts ex. "May I use your pen" "May I get the door for you?"
"Are you able to exchange these into 10's and 20's please?"
"Are you able" again is a very polite and formal way of phrasing this.

When you want to change bills into 20's and 10's, you can ask in the following ways:
-Can I have 20's and 10's please?
-Can I exchange this bill into 10 and 20 dollar bills?

When travelling or going to a foreign country then it is usually better to have smaller notes to pay for things as sometimes bigger notes like 50s/100s are not taken in smaller shops
If you want half of one and half of another then you can also say 'split equally'
by saying '10s and 20s (tens and twenties)' they would know what you meant

First example expresses wish to have two halves whereas second example asks for half in 10s and the rest(meaning other half) in 20s. Both acceptable and easily understood. Both examples formed with modal verbs followed by main verb creating questions. Could get. and. Would like

Being specific on how much you would like exchanged would be helpful to the one exchanging the money for you. Add please to show good manners.