Costly/clumsy if the two of the devices already exists and already talks RS232. That means that the OP have to convert RS232 -> logic level -> RS422 -> logic level.

I had to write software once for a system where up to 16 processor boards were placed in a 19" rack system - each board had a RS232 port connected together. So the PC could send data to any of the boards by supplying an address in the message. And the individual boards had a digital signal (summary alarm) they could draw to inform that any of the boards needed service. Most funny - any slave board talking at the same time resulted in a deadlock requiring power cycling. System has been in operation for 20+ years and works well - but not worth the trouble to do something like that unless absolutely 100% totally cornered.