If Mark's approach doesn't work and you HAVE to do this you may need a device to function as a "traffic cop".

There are any number of very small computer devices available running some variant of embedded Windows. With three serial ports and a small program you could achieve what you need.

The program would in essence have to monitor for commands from either "control" port and pass the command to the controlled device, wait for the reply and direct it back to the device which initiated the command via the "control" port on which the command was received.

This would prevent commands that might come in on both "control" ports, at the same time, from "colliding". One command would be delayed until the other had been processed.

Depending on the complexity of the commands and responses it could be a bit tricky but it could be done.