The only other option you have is to see if you can find a dev to compile Cyanogen for froyo for the Optimus S and then streamline it with Reborn features. I don't think any CM port was done for our phones for the froyo build though. I could be wrong.
One way would be to flash a rom and do a nandroid backup from recovery. After that, flash the other rom, get it setup how you like, and do a nandroid backup. Then at any time you can boot into recovery and cash the appropriate nandroid. And remember, the nabdroid saves all your stuff so settings, apps, etc will all be as you left them!
It's possible with phones like the HD2 or the HTC hero, and the Nook. I'm not sure if anyone has ported it to our phone, but I'm sure it's a possibility if someone wanted to spend the time.
i don't know about duel booting roms but you can duel boot into linux distros theres a thread about that here and one of my friends duel booted into debian linux