I am getting a home built that has two separate water usage areas separated by a wide living area. I am likely going to go with propane tank style heaters since the tankless units (and their installation) cost so much. I want to do a 40-gallon over the master and kids bathroom. Then another 40 gallon on the opposite side of the house for the kitchen, laundy, and guest bath.

What I want to do is fill the one over the master with hot water coming from the kitchen tank. However, the kitchen would also supply, in parallel, all of its own dedicated taps (kitchen, laundry, and guest).

The reason for this is that I am going to have a 6ft garden tub and 40 gallons alone will not fill that with hot water. Also, if the whole family runs showers together, this would enhance our recovery time. It is once in a blue moon that we will actually use the tub, so it does not make sense to put a huge tank over the master if i had a second tank. The downside is that showeing will drain the tank for the kitchen, because it has to make up the lost water of the master tank. The upside is that the master tank never really is heating the water, it is just maintaining it.

I had considered a single huge (75 gallon) tank, but one side of the house of the house will have lots of lag (unless I use a recirculator but that seems like a bandaid fix). Or I had also cosidered a point source type system specifically for the garden tub, but that is another added expesne for the heater, the huge copper wirng for the extra circuit, etc.

Any thoughts on the best approach to this. I am trying to minimize my faucet run times to get hot water and also minimze costs and complexity. Would it be better to do a 30 over the master and a 50 over the kitchen? Or some other combo I have not thought of? Thanks!!