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For a building with two boilers serving separate apartments, I'd like to simplify my design by having only one auto-fill and backflow preventer, and only one expansion tank.

These are combi boilers using a primary/secondary loop for each.

A single supply could fill both loops--it's all coming from the same cold water line anyway, right? The only risk would be mixing of loops under weird pressure conditions--should I use check valves? Is this common practice?

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What you asked is not common practice. If the boilers are not tied together in any way, common practice and that which is acceptable is to have one common back flow preventer and a separate PRV for each boiler. There is nothing to gain by attempting to use 1 PRV to supply water to both units.

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If those are combi, there's no need for expansion thank unless the heating system is quite large (usually embedded thanks can accomodate a system of up to 100L of primary water). Expansion thank on secondary circuit (DHW) are not needed because, heating water only when required, there will be no pressure build-up in no usage time.

One backflow preventor is possible (put it just after the meter), but seen the price (10€ at most) using 2 pieces will not be a big issue

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