How do I install mountain lion but keep snow leopard as a bootable os

Question

I'm caught in a vicious circle.
I need mountain Lion to be able to use iCloud and to be compatible with a couple of apps on my iPad. However some of my major apps on my mac Pro won't work under Lion or mountain lion.
SO the answer is to have both Snow Leopard and Mountain Lion on my Mac Pro.
Does anyone know of any instructions on installing Mountain Lion onto one SSD, whilst leaving Snow Leopard on another, allowing me to boot from either.
Also - any advice on the cleanest way of installing my apps under Mountain Lion (those that are compatible). I'm confused to how I can install my apps twice, especially if I have all ready registered Unser Snow Leopard.
Thanks

Answer 1

I'm assuming that you currently have an SSD with Snow Leopard on it, and another SSD with no operating system. Is that correct?
What I would do is use the Snow Leopard DVDs to install a fresh copy of Snow Leopard on the SSD that currently has no operating system, and "migrate" your data and applications from the other SSD. (alternatively, you could use something like Carbon Copy Cloner to duplicate one SSD onto the other - but that's probably only an option if one SSD is completely blank or you're willing to erase it).
Then upgrade to Mountain Lion on the original SSD.
When you're done, hold down the option key when you start up to select which OS to use.
For many applications, it won't matter which SSD they're on - you'll be able to run them from either system without worrying about where the app is installed (each will have it's own preferences, stored on the current startup drive). But some applications are more complex, and rely on additional files that are scattered around the Library folders on the startup drive. Since you migrated your applications and data, you should have everything that's needed on both systems, including license keys, etc. I did this for a little while with Quicken, and I actually left the Quicken app on the Lion drive - it worked just fine even though I started up from an external Snow Leopard drive (I skipped the migration step entirely, and reentered my Quicken license code when starting up from Snow Leopard).
How successful this will be depends on exactly which applications you're using. Most of them will be OK, but if you're using something that has some esoteric copy-protection mechanism, you might run into a little bit of trouble. Consider upgrading the "new" Snow Leopard install to Mountain Lion, if that's the case, so you're original system SSD will remain Snow Leopard, with no changes.