I would be really careful about heading down this path. One thing you need to keep in mind is that during patch installs, OS updates, etc. there can be a LARGE temporary spike in disk space requirements. This can easily be on the order of a few hundred MB on 11.x.
This 'extra' space is needed because when updating shared libraries and other files that are in use, the system renames the old one rather than deletes it, then deletes the renamed file after the next OS reboot.
During that window, you need space for both copies of all of those files.
Although the SD tools will in theory do the appropriate disk space analysis and warn you if you're about to try something impossible, and those checks usually work well, they are only estimates. I have seen cases on smaller disks where the SD disk space analysis passed but the system ran out of disk space in the middle of the update, leaving the system in close to an unbootable condition.
You would likely have much better luck if you striped the two 9GB drives to form one 18GB drive. That would give you enough space for reasonable partitions, some swap space, some crash dump space, a little breathing room for installing later patches, etc.
If you do try this on one 9GB disk, you would probably want to minimize the number of logical volumes, so that free space isn't locked up on an lvol where it can't help with the space needs of another lvol.
Another consideration is how much software you plan to load onto this system beyond the original OS media.
Of course another consideration is disk reliability. You don't state the age of the disks, but it seems a reasonable assumption that any 9GB disk is quite old, and may be nearing the end of its expected life. Depending on how painful a disk failure would be to you, it may be wise to think about replacing that disk before doing the OS install.
