Vithal:

As I can see, you are forced to use only two lines, so you can
not use I2C or SPI - bus-communication, otherwise, thats
an common way to perform an Master-Slave communication.

Maybe you can take a look at CAN-bus principals, I think that
goes by two lines, in some sort of master-slave relationchip.

However, It seems to me, that the best solution are that you
start develoop your own protocol with identities, and polling systems
or something similair to slotgy ahloha depending on baudrate
requirements.