1992rs/ss

so I have one of the monster fm transmitters for use in my car and on my boat. it plugs in on the bottom so I was wondering if I also used one of the FM transmitters that plugged into the headphone jack as well as the monster unit I have, would that boost signal or would the radio just pick up the strongest signal and that is it? I know everyone will say hardwire it in, but I want it so when other cars or boats are parked near each other, we can combine stereos for more volume. anybody know much about this? tried it? thanks.