Place an object in the prison, it could be a chair, a soldier, a gamelogic, etc... name it prison_obj.
Then make a trigger with the following in the Condition field:
F1 distance prison_obj < 15 && F2 distance prison_obj < 15
That will make the trigger fire when both F1 and F2 are within 15 meters of prison_obj.
For the carlock issue, I seem to recall a fairly recent thread on this, search the forum - should be simple to find.