Using one program Application on both Lion and Snow Leopard

I have to constantly reboot/restart between Lion on my New Thunderbolt Mini and Snow Leopard off and External Drive. What is the recommendation for using programs (which are built to run on both) such as iwork pages. Should I use one program off the Main Lion Internal HD? or as I have now - Maintian two programs, one on each hard drive. I can see Lion is having difficulty as it can see the other program and sometimes it launches the version on the SL drive. When in Snow Leopard the OS cannot search and therefore not register items not included in the Main Boot Drive.

I am a single user, so I'd like consistency in maintaining templates (and now version!?!) when I want to draft a letter and can't waste time to reboot. All my documents on both OS's are directed to a 3rd Drive to effectively keep my sanity. I was syning the preferences previously for mail and passwords,etc but recently the functions of Lion have expanded so much beyond SL as that I had to swtich off this sync to avoid conflicts.

Should I keep using two programs or when available use a single version. Noting this wont be on the boot drive of one of them. If I go to one. Can I keep the other on its boot drive in case of transporting it to another machine. Is there a way to park it so it's not referenced to use when I launch by opening a file? I though about zipping it? What happens when a version update might excel beyond SL? The current programs known already have 2 versions residing on each boot drive. If I go to 1 and a software update comes along, that would be a problem.

Any and all experinces and suggestions are appreciated.