Any standard installation of A/UX also installs a fully-functional version of MacOS, separate from the boot partition that installs on the "Macpartition." The latter is not where the real work is done, and while it is very definitely needed, you really just let it do what it does and then forget about it. It need only be a tiny partition, as you will not ever install anything else in it.
If it's A/UX 3 you install, then further to the boot partition, you get MacOS 7.0.1, which installs within the UNIX file system on the main partition. There are thus more than one MacOS System folders in a standard A/UX 3 installation (and a new one for every user, I believe, though presumably these are mostly a matter of preferences and such). You can't upgrade any of these to 7.1 or 7.5 without bad things happening, but as it stands the system is very stable and on an SE/30 you will be able to run most anything that an SE/30 is meant to be running MacOS-wise. A/UX will also get you beyond the dirty ROMS problem of the SE/30.
There is a write-up about the different System folders in one of the main A/UX 3 manuals available online from Apple.
A separate, second installation of MacOS (say, 7.5) on a third partition should also be perfectly possible, but as the stock MacOS installation of MacOS is fully functional, I am not sure why you would want it. I suppose that if you wanted software available on the machine that required 7.1 or 7.5 to run, like AppleScript, it would make sense. I once had a similar arrangement (with 7.1 or 7.6 - I forget) briefly in a Quadra 950 but in this case there were two separate drives. I soon reverted to A/UX only as, given my interests and the other machines available to me, I did not really see the point of the separate MacOS installation.
