As has been pointed out already, RS232 defines an electrical (and mechanical) standard for serial data transmission.
It is only intended for point to point communication, or as you put it "two devices".
If you want to communicate from one master device to several slave devices, one way to go would be to use RS485 rather than RS232.
RS485 is a two wire communication system, when there is no data being transmitted, all devices are in receive mode.
Each slave device has an address (how you organise this is up to you, RS485 only defines the electrical standard, not the message protocol), when the master device (maybe a PC) wants data from a slave device (an RFID reader), it polls the slave and the slave responds with the appropriate message.
Whenever a device (master or slave) wishes to transmit, it has to:
turn on the transmit circuits in the interface,
send the message,
turn the transmitter off and return to recieve mode,
listen for a reply if one is expected.
Have a look at MAX485 (Google it), an IC which implements an RS485 interface.
JimB