Yes, you can have two version of windows movie maker on your computer.
But you don't mention which system operation you're running. Let me guess, you got wmm 2.1, that might be windows xp system. That might take you time to find windows movie maker2.6 for xp. The online download of windows movie make 2.6 is for win 7. Making sure you find a xp one and installed to your computer.