You sure can. You need to of course name the installation folders differently so you don't get them mixed up, but it can be done. There are problems though when downloading stuff as Content Manager will go for the most recently used TS12 install and put the content there. This can cause a lot of confusion. Shane Turner has written a utility that will resolve this problem.
I have the situation you want.
I have a 49922 build and a 61388 build next to eachother with the only different being the base folder name.
You can use Content Manager of both installs and this will download the assets to that specific install without any problems.
If you download from 3rd party website and just double click on them to install, they will pick the latest install. You should be able to manually import via CM if you want the stuff in the other install.
