Typically, you lose three or four levels' spell progression on either side of the rail for taking Mystic Theurge (usually making it a bad idea). However, this being D&D, there are usually ways around that sort of thing. So...
1) Is there any way to take MT all the way such that you can get 9th-level spells on one side or the other by level 18?
2) More importantly, are there any other classes that can let you continue your dual-progression once you reach MT10?