Tankless Water Heater Q?

I had a Rinnai gas tankless water heater installed in my new construction home 10 years ago. Haven't had a bit of trouble from it (so far). Anyway, they said my heater was good for 8 GPM and that I could run 3 appliances at the same time (2 showers and a washer etc.) at the same time. So far, they're right. I've never run out of hot water in my shower no matter how many fixtures it was supplying.

So to answer your question, I think you could run 2 sources with your 5.3 GPM heater at the same time and not run out.

The issues that boutrous brought up are a completely different matter. Good luck.