The problem you have is that two devices need to transmit on the same wire to the PC. If you are certain that only one device will be transmitting at any given time, you can solve this with 2 diodes and a pulldown resistor on the PC Rx signal, or a diode and 22k in parallel from each Tx line to the PC Rx. The inactive decvice will then function as an active pulldown. The PC will then transmit to both units at the same time, and get an ANDing of the Tx lines from each device.
This will only work if the devices don't answer communication destined to the other device.
The value of the resistor will be a bit depending on the baudrate you use, but 10-22k should be ok.