I think Stuffit will actually do this, and it comes with the Mac. I believe it will also rejoin split files, but if not there is a Mac app on the Hjspliter site that will do it. The best app for both though is an OLD OSX app called "Rosetta", not to be confused with the PPC to Intel Rosetta. It's been out of circulation for a bit, but it was freeware and if you like I can get it to you. It still works great and is almost the only thing I use for this sort of stuff.
Check out Version tracker and type in splitter. Several apps come up.