Delete all of your extra regional/language files from the OSX install along with Garageband and its files. That will clear up quite a bit of space. Do a forum search on deleting language files, there are a few threads out there.
My answer is no. If you find a way to do it, you'll probably mess up one of the OS's. There might be a program that could help you, but I doubt it. I wouldn't risk it.. If you do go ahead and try something be sure to back up.
EDIT: Quick google search turned up nothing.. You'd need a partition magic equivilent.