If you realy find your unit will not handle it and it operates
correctly look into a Rinnai tankless, you cant run out of hot water and
will save in its operation as it is more efficient than regular water
heaters. But first try what everyone has mentioned. Shower restrictors
and showering first before running a tub, and put a time limit on
showering, hot water can easily use 30% of your gas usage even if you
live in a cold climate.