I considered this, too, unfortunately I can't find my old threat anymore, seems to be lost in this board. There was a complete list of all 2-trackballs-games in it, maybe I can find the list when I'm at home.
I remember Rampart which even supports 3 of them, and there were some shooters and many sportsgames.
Well, I don't like sportsgames at all, so for me it`d have only been for Marble Madness & Centipede84
Don't know the name anymore, but it was a 2-games-prototype featuring a horizontal Centipde-Version you could play with 2 players.
As you can imagine I left it with one trackball, it's not worth the effort for me. If you like sportsgames though, it may make sense for you.