One wire (+GND) serial communication. How to have a correct coupling?
You can connect together any number of pull-down outputs and a pull-up resistor with no danger of short circuit. There needs to be scheme that avoids two outputs pulling down at the same time, but that is harmless if it happens. A possible scheme is for one transmitter to be Master which sends a 0 pulse every 10 seconds. All the other transmitters are Slaves who know their distinct addresses so that Slave #1 may send something 1 second after the Master pulse, Slave #2 may send 2 seconds after,.... and so on. This example is deliberately slow because the data rate will be limited by the capacitance of the signalling wire, which we don't know.

The one-wire protocol device that KyranF mentions is clever because it manages to get its power from the same line that it occasionally pulls down.