I played MW3 last night and used the split-screen feature to let my girlfriend play also. I noticed on very big problem though. I spent 2 prestige tokens(yes, already!) on extra custom classes, so I have 7 classes. In split-screen though I could only pick from the top 5 or the default classes. Is there any way to access my other custom classes while using split-screen?
There's no workaround for this currently; it's simply a limitation of the game. It doesn't seem like it's a priority for the developers to make this work properly in splitscreen, as they are still doing active bugfixing, but haven't addressed this issue.
You can see what's still on IW's "To Do" list, along with what they've already addressed, on this page.