As above, and the other problem with trying to use a hardware usart with simple radio modules
is that in the absence of any transmission , the receiver outputs continuous random data transitions
which are not at any constant baud rate , and this will cause the usart at the receiving end to continuously
generate overrun and framing errors, which must be cleared before it will work properly.
To do what you want using radio needs intelligent Transmitters / receivers which will simulate correctly a hard wired
connection.
Look at using Xbee modules, some of which can do this.