I'm developing a game by myself for the jam and doing the artwork for a second team project, but it only occurred to me now that it might not be possible to post more than one game.

Can you tell me if it's possible?

yes

You cannot work on more than one team, 

And working on a team as well as solo is not what many people will consider doing. If you think you can its fine to submit more than one project.

You cannot post a game by yourself and then also work for another team's game. You are still considered a one-man team, and each person is allowed to work on one game and one team only.