Can you or anyone else reading this recommend a good tutorial for splitting serial time interrupts?
I don't believe that that is what was suggested. Re-read what was suggested.
You can't control how the reception of data. You can only control whether you are paying attention at the moment data arrives. Given that, it seems obvious that you don't want to not listen to either device for any extended period of time. Given that, it seems clear that different hardware (a Mega with 4 hardware serial ports) is in your future.