If all you really want to do is to receive AM broadcast signals, it's hard to do better than just about any car radio connected to as long a piece of wire as you can manage for an antenna - even 25 feet or so would do well, but if you are in the woods, stretch out 100 feet or so as high in the...
Most likely you would blow the receiver in the non-transmitting radio. Best to separate the antennas to reduce overloading the receiver of the non-transmitting radio when you transmit with the other. Same is probably true about your entertainment radio antenna as well. I once had 7 antennas...