ExplainIng how isolated rs232 interface works
When comms is low, T2 is closed and T1 is open, T2 is needed to provide the path to ground, and brings Rx active through T2. 
When comms is high, T2 is open and T1 is closed the opposite occurs.

You need 2 transistors to get this switching action in this case. 1 could work in a theoretical application, but the amount of other passives involved would probably degrade the signal past the point of no return. The same transistor is likely used to allow for higher baud rates, as the switching time will be constant for each.