Resolution has to be lost. Discounting ADAD conversion errors, your input from the one board is in 1024 distinct steps, but the output from that board is 256 distinct steps. and it isn't 4.5v/256 it is the cells voltage above relative ground that is sent.
I assure you that you wont regret sorting out serial communication
They have a two port example for software serial built right into the arduino ide.
Instead of pwming, you just write the converted adc values over serial, with no loss, plus you can control other serial devices.