Go to track central and download a Supercross track.
Then go to Multiplayer - Local and choose the track you downloaded. You can remove all tracks and just have the one you downloaded. Then connect one more controller and play the track.
You can also do this under Xbox live - Create private race and invite a friend if you don't have 2 controllers.
Update. If you can't find any good track search for "1 Sec Supercross" it's very short Supercross track. You can press X to search and enter my gamertag demonWhistler as creator to find it. It's case sensitive so one capitial W in the middle.