This won't work.
You can only have one driver on a serial line; your scenario has two drivers on each line.
You can use the arduino board as an rs232/USB adapter if you pull the ATmega chip first.
Also, rs232 has different voltages and levels than TTL devices (like the arduino). If your custom devices has true rs232 voltages (e.g. it has a DB9 port) you'll have to have a bit of hardware between it and the arduino board to convert those signals to TTL levels and voltages.
-j