With all due respect to the two gentlemen above, I've been doing Multiclips with dissimilar timecodes in FCP6 and FCP7 for years because I have no means to jam timecode. As long as all the other attributes (framesize, framerate, codec, etc) are the same, all I do is find a good sync point and mark an IN point on the clips, then use the Make Multiclip command using specifying IN Points as the sync.
The IN points aren't necessarily your chosen IN points, they're just a sync reference in Multiclip. You can create new IN for the Multiclip after it's in the timeline by just sliding it down the timeline and pulling the left end of the clip back towards the timeline start.
I learned this technique from Steve Martin (founder of Ripple Training) when we shot his son's baseball game with three Canon GL-2 cams (no provision for timecode sync) tback when Multiclip came out. We published a short tutorial on it, '06 I think...
hope this helps