does the /etc/fstab in the SuSE install have an entry for the Ubuntu? I'd imagine that the Ubuntu /fstab, might have a SuSE entry as it should have detected it when the Ubuntu was installing.
Oh and yes it's perfectly feasible to have read/write access, you'd have to make sure about permissions though.
Last time I had multiple Linux installs, the only downside of it was that because I used the same /boot, /swap and /home, if I'd created any shortcuts etc, in one distro, I had to make sure that I had the corresponding packages available in the other. Which was a pain because I'd remembered to do that with images, icons etc, but not always the software itself. Plus, as I had debian and mandrake it could sometimes be a nuisance, debian has so many more packages available than mandrake!
In the end, I just said bollocks to all that, and settled on gentoo. Much more satisfactory.