Me and my friend recently set the WR for the 2P run in Super Mario Galaxy 2. However, while I can select the 2P category, it only lists me (the account holder) as the runner under "Player 1", but doesn't let me list my friend as Player 2.

People told me it is possible, but is it enabled in the entire site, or just for specific games? It just sucks that we both did the run, but only I get the credit listed :/

It's game specific, decided by the game's mods. You'd be more likely to get a response here: https://www.speedrun.com/smgalaxy/forum