I have a 5 bedroom, 3 bath house that is served by a single 80 gallon hot water tank that needs to be replaced. Are there any advantages to replacing with two smaller tanks that are "zoned" to split functions? Any thoughts?
It can be more efficient, yes. It depends on the efficiency of the new tanks, contrasted with a new 80gal tank, your water usage patterns, etc, etc. If you can easily re-route pipes, it can be advantageous to split off the showers from the rest of the house, as you can set a timer on the tank for the showers, so that it comes on a few hours before your showers, then off soon after, so you aren't heating the water all day. Make sure to use an external insulation blanket to increase the efficiency even more.
Another item to consider is a tankless heater, but if you don't have NG run to the house, you'll need some heavyduty wiring to support an electric tankless to support enough load for a large household.