I have recently bought a house where the past owners installed a gas Bosch tankless water heater in place of the 50 gallon gas water heater. They left the 50 gallon heater unplumbed but in place.
I would like to know if I can plumb the 50 gallon in series with the tankless, in that order.

The problem is that when we have guests the tankless water heater can not produce enough hot water for two showers at a time.

My thought is to plumb the two units in inseries but also have shutoff valves placed which would allow me to bypass the 50 gallon heater when our hot water demands are less.

Basically to go form the two units in series to just using the tankless.
Is this possible??