Are you certain that both fills say "water"...that one doesn't say "waste?" The two words, both starting with WA, look a lot alike if you're not paying close attention...and being just etched into the flange doesn't make 'em easy to read.
Hmmmm...never seen that before unless an owner did it. Seems to me that if there's room for two 28 gal water tanks, the designer or builder should have been able to find room for a single 60 gallon. Would have cost less than two complete systems too.