Been a while since I used it. But if memory serves, ffmpegX has a very flawed AVI splitting. Files that are split can't be joined later. To be safe I'd normally split the AVI/MKV on Windows, because let's face it, those formats have barely rudimentary support on the Mac. To date there isn't a single program on the Mac that can do half of what VirtualDub is capable of. I'm speaking of framerate conversion, video filtering, AC3 5.1 muxing, etc. Not to mention precise keyframe splitting.
If you don't mind X11 avidemux fills in some more of those gaps.this has been one of my biggest gripes so far... can't find a replacement for virtualdub. I don't want to have to boot up/virtual XP for it, but it's getting to that point.
D-Vision 3 is the best option. Go to the Tools menu and select 'segment avi file".
It's been a while since I split anything with it but I remember I had to choose a fairly unintuitive option to get the kind of split I requireddvision has been hit or miss for me unfortunately
I'm sure X11's enthusiastic admirers are few and far between but it's marginally better than rebooting into Windows...well, some of the time at least.and as for x11... I try to stay away from it.