Heating two indirect hot water cylinders

The normal solution would be to make it a S+ plan system with a two port valve for each zone. So in your case, one heating zone and two hot water.

So similar to:

I'd plumb the heating coils in parallel and connect the cylinder stats in p= arallel, so if either calls for heat, both get it. This means one will over= shoot the target temp, but only upto at worst the temp of the primary circ= uit. The cooler cyl will be heated more than the hotter one, since the delt= a T across the exchanger is greater.

NT

The risks of that could include one cylinder getting upto 85 degrees depending on the flow temp. This is risks to the users of the water if there are no blending valves, and can also make scaling of the cylinder more rapid and pronounced in hard water areas.

You will need valves to balance the heating water flow between the two tanks in exactly the same way as radiators.

It will have quite a long recovery period. (ie warming up from cold).

How about using the closed live output from one of the valves to trigger the second valve (via the thermostat) - that way you could give one tank priority over the other, so from cold you could use one bathroom first.

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I guess it depends whether the OP wants water that hot or not. I had it hotter at one place to get enough capacity

NT

That is what I would do with the addition of another channel of time control for the second cylinder. Keeping a cylinder of hot water hot "just in case" is a good way to burn more gas than you have too. The heat time of a single cylinder from cool shouldn't be more than 30 minutes, it's not difficult to "pop the water on" 30 plus mins before you want to use it. If if both cylinders are cold the reheat time might be an issue but again a simply manual priority switch (set the relevant programmer to OFF) solves that.