Greetings,
I'm designing a DHW recirculation system for a building that includes three separate apartments. The hot water pipe will split into three separate branches right near the DHW tank. The first branch will serve one kitchen which is very close and will not be recirculated.

The other two branches (about 50' each) will be recirculated. My main question is whether is should combine these two branches into one long (about 100') recirculation loop or if I should have them meet, share a common return pipe to the DHW tank and install a balancing valve to control flow rate through both branches. The building's layout makes it convenient to do either.

The first branch will serve two full bathrooms, two kitchens, one half bath, and a washing machine. The second would serve one half bath and one full bath.

I have no experience with recirculation systems and I'm not sure if I am just imaging that the common return setup would even be a consideration, so let me know if I'm overthinking this. The only real advantage I can imagine to it is that the hottest water from the DHW tank will get to all the fixtures sooner and perhaps allow a recirc pump schedule that turns on less often. Otherwise, it is more costly and complex to install, but I'm mainly thinking about the lavatories at the end of the long loop that may never see water direct from the DHW tank. What if they are used right before the pump kicks on. Will they often be slightly cooler that the other fixtures? Any thoughts or advice from someone with experience would be appreciated.

if you are using only one pump then you have a common return no matter how you pipe it..
everything is returning to that one pump it is just a matter of where the returns join.
anyway, without seeing the piping layout it is difficult to say how you will balance this or even where the extra tees for recirculation should be located. generally the tee is located between the last 2 fixtures that use hot water.
every extra tee you install will need a balancer..

The water will go whatever route has the least resistance.

The way I've seen them designed, the return goes through the tanks pressure relief valve. Essentially, I could run one pipe with no tees (except at fixtures) from DHW out to pressure relief back in. Or, I could tee off immediately after the DHW out, to make two branches, and tee them back together where they would meet, and then go back to the tank.

The more I think on it, it seems like a silly idea, but any reason to do the later?

you don't return to the relief valve port. you return to the drain valve location at the bottom.
as i said, it is hard to advise how to pipe the recirc line/s with out seeing the actual piping layout.

One single loop.

Well insulated by a PRO insulator.

Pump to the tank drain tapping.

Aquastat controlling the pump.

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