You can run them both at the same time. What you cannot do is have two versions/copies of LR open the same catalogue.
Which brings up another question. (Sorry no intent to hijack the thread.)
I was thinking of adding 2 iMac's in my gallery for my workshops.
Both with CS5 and LR3 installed.
Only loading images for the students to work on,no catalogs.
Will the 3 computers with the above mentioned programs work at the same time? My Mac Pro and the 2 iMac's.