Existing 50gal nat gas WH in home (kitchen/DW + tub/shower bath + laundry), with basement 'mother-in-law' rental (kitchen + shower bath + laundry), needs to be replaced.

Seems like need more than 50 gal, but next step is 80gal.

2-40gal is much less initial fixture cost than 1-80gal.

Make sense/advantage to go with 2-40gal in series (or whatever piping setup is best, not a plumber) ?