If you mean twisting in order to have the flat belt back go over flat pulleys, that's great except where you attempt to twist a length of belt that varies in span, e.g. between motor and carriage. The twist could end up very tight. Or if you're twisting so that the crossed belts don't collide, you would still need some vertical separation in order to prevent abrasion.
If you have a look in my "CoteXY build details" or the "after Hbot go CoreXY" thread, page 10 I think, I propose running the belt inside-out so that the majority of pulleys run on the belt back. You just need two cogged pulleys plus the motors, and there are a couple extra pulleys required. It's not tested yet though.
