The upstream water htr has a sidearm heater from my outdoor stove and the convection heating will not keep up with the number of people I now have in the household.

In this case you can go with the takagi tankless water heaters. They have different type of sizes for different household sizes plus you can connect these ones together and use it as one. You can follow this link to see how it is. TAKAGI TANKLESS WATER HEATER (http://gpfsupply.com/index~ID~,Plumbing,Water.Heaters,Tankless.Water.He ater.~path~to~ds~dept)

You can connect 2 heaters in series or parallel

In series, the one will feed the other, so the one hit with the cool water will start firing, feeding the second with the burner off until the first is too cool, then the second will fire until it too can't keep up. If the flow rate is more than half the total of the two, eventually you will have both firing.

In parallel, both runs until one is not hot enough, so it starts firing while the other feeds hot water. Then both fire when half the content is gone (because the hot is now mixed with cold.)

Neither has anything to do with efficiency of the burners, it all depends upon how fast you use up the hot water. If you have a ****** use where more than half the hot water of total tanks is used up, either way both will fire up.
It is just that if the smaller is fed in series by the larger, the larger will start firing a little later. If the larger is fed by the smaller, the smaller will fire sooner.
If in parallel, balanced flow, so the same percentage is drawn from each, a larger amount is drawn before either has to fire.
It all depends upon how much you want to draw at a length of time. Both burners firing at the same time gives you more hot water for draws within the firing rate, Thanks.

John