If the device is running on the serial interface and not TCP, then you can just have one Master router and that's it.
If you're running on TCP and want to read/set data of the second router, then both router must be set as Master and Slave.
About RX and TX:
Yes, it means that you're connecting TX (transmitter) to RX (receiver).
To give a credit to author of this article I'm going to link it here, so you could read it: