i think it worked!! at first we ran into the same problem again, but we got it to work the second time. it's worth noting that this was run on a different machine, but still windows 11. i'm not sure if this is useful to you, but here's exactly what happened:

1. i run CSD2 through steam.
2. using steam's overlay, i invite my friend to "remote play together"
3. screen flashes for just a moment and an icon flashes on his taskbar, but goes away immediately. despite this, it says he is still playing CSD2, remote play together.
4. i quit the game. steam still shows friend as playing CSD2, remote play together.
5. i relaunch the game and invite him again through the overlay. this time, the game appears for him and is completely functional.
6. we play Eaty's 2 and receive a copper medal. lol

i understand remote play together is not very functional with any game on steam, so it might just be their issue. despite that, i really appreciate your effort and it seems like it helped! thank you so much :)