There is no reason why you can't have RS485 and RS232 working in parallel with the appropriate constraints. Say we have the PC as the master,when it is sending data this can go out both the RS485 and RS232 interfaces with no problem, in the case of receiving, things get a little more tricky. If the receive signals from both the RS485 and RS232 interfaces are 'or'ed together (or 'and'ed depending on the logic) receive data on either the RS232 or RS485 interfaces can be routed to the PC receiver. But if devices on both the interfaces respond at the same time, the data will be corrupted. You need to ensure this does not happen by the use of a protocol.

As to why you might want to do this is questionable though. It would jusr be easier to use RS232/485 converters where necessary.

On the PC side, I would recommend a ADAM4561 usb->232/485 converter from Advantech. These take care of the RS484 tx/rx control in hardware making life a lot easier on the PC side of things.