I read on this form last year that if you connected Mega(1) serial1 port Rx to Mega(2) serial1 port Tx, and Mega(1) serial1 port Tx to Mega(2) serial1 port Rx, one serial port would draw too much current when communicating to the other Mega, is this true? I am hooking up 3 Megas together to share information for my home Bugular alarm, which is all hardwired. Two of the Megas run the remote keypads and displays. The third manages all the windows, doors, motion sensors, charges the battery and alarms - I/O.
No true for a short distance, they would almost draw no current at all.
When you connect long wires to the serial RX and TX pins, many bad things can happen. Voltage peaks could damage the Mega board; The communication could be too slow; and it is indeed possible that the TX causes current peaks. The RX and TX signals are they are, are not suitable for long wires.
There are many ways to use long wires. Line drivers, chips, filters, circuits, protocols, and so on.