Possible, but when you install X, it will "hide" the earlier application in a new folder, at first it may look like it wiped it out. But it should all still be there and still work, if it worked before you added 'X". This is what i heard from people using FCP7 and adding X. Dig out the icon for FCp6 from th folder and put it back in your dock and now you should have both apps available. Though I would NOT try to run them at the same time, close out of one all the way before opening the other.