Let's say we're writing two programs that uses both UART bus. The program A uses UART at 9600bps baud rate, and the program B uses UART at 115200bps baud rate. Can both of them operate concurrently and independently without affect each other?
No, in general they can not. An RS232 based serial link is not a bus, it is a single node to node connection.
There is something called RS485 which is used for multi-node connections. However RS485 only operates with a single baud rate for all nodes.
You could try to program your solution but would have to cope with collisions yourself. This might work if you can write the software at the Pi end and each of the devices you want to talk with.
However if one device only transmits and the other device only receives it might be possible (I'm not sure if the Pi end can cope with different receive and transmit baud rates).