TestComplete 11 and TestExecute 11 can be installed side-by-side with TestComplete 10 and TestExecute 10, but they do use different licenses. Having them installed on the machine won't affect anything (other than taking up space).
Make sure that you have activated your TestComplete 11 and TestExecute 11 licenses. The version 11 licenses, similarly, can be activated on the same License Manager machines as your version 10 licenses. (If you put the licenses on different machines, make sure your clients know where to look for them...)