Not only can you run both versions, I highly recommend it. If you want to use your old Python scripts, you need to have Poser 11, since Poser 12 won't run them (yet).

There is a free 21 day trial, so you can see for yourself. One overwriting the other shouldn't be a problem, since Poser 11 installs in a folder called Smith Micro and Poser 12 installs in one called Poser Software.