I routinely have FileMaker 11 and 13 open at the same time on my Windows 7 machine. So far, no problems have occurred that I can trace back to having both open at the same time. This has typically been the case for different versions of FileMaker.
You can run multiple installations of any application using Sandboxie. Useful in case you need to run 2 version 11's at the same time on the same machine.