Unfortunately you can't send and receive at different baud rates simultaneously because the transmit and receive use the same serial clock (Timer2)
However there are 2 solutions to the problem:
1. you can set the timer2 for 4800 bps for receiption, when you need to transmit you disable Rx interrupt set baud rate to 1200 send your data, when finished you set it back to 4800 and re enable Rx interrupt.
2. if you need to be able to send and receive simultaneously use a derivative with two UARTs set one for receiption @ 4800 and the other for transmission at 1200
Regards
Mahmood