adwsystems:
If they outputs are tied together to the input it works. That indicates to me they are not both trying to talk at once.I also tried a diode or gate (web says pull down at OR gate output). No joy there too.
You wired the diode ORing wrong, then, post schematic. Did you put a pullup to Vcc on on the RX line? Did you put the diodes the right way around? band should go towards the RX lines (since it's active low).
An OR gate will not work. You need an AND gate, because a TTL serial line is HIGH, not LOW when is is not transmitting. If you OR together two lines, one of which is transmitting and the other of which is not, the output will be constantly HIGH because the idle line is HIGH - HIGH OR'ed with a signal is HIGH - no signal out.