or i think what some people do is in the settings of winrar is you set a certain size of files (so it splits up one 100mb file into 10, 10mb files) then you can compress it again so its compressing those 10mb files. im not sure if its compressed better but i've unzipped lots of stuff like that
I had a look at the compression ratios on this a while back and while I don't know the details, I'm pretty sure that splitting it into different files achieves a slightly worse compression rate than just one individual one - with multiple files you've got the overhead of what data is going into what file which will take up more space.
Most of the uses I've seen for this sort of thing are on sites like rapidshare where someone wants to upload a 2GB file and there's a 100MB limit per file - they'll put it up as a split rar archive.
If this is for backup purposes though does it have to go onto a DVD or 2? Why not just put it on an external hard drive or similar? I used to have one that was purely for backups, they're pretty cheap these days and it worked quite well for me.