I know this is four months late, but you could solve that with two inserters between your warehouses and some circuitry. Have green wire connecting all of your warehouses together, and going to the input of a arithmetic combinator set to divide whatever your warehouses store by the number of warehouses, output A. Put two inserters, one facing each warehouse, between all of your warehouses, and a decider combinator next to each inserter, with the combinator output wired to the inserter, and connect the output of the arithmetic combinator to the input of each decider combinator. Set all of the decider combinators to (item your warehouses store) < A, output green, then set all of your inserters to enable if green > 0.
This is basically doing the exact same thing as equalizer chests. The arithmetic combinator calculates the average number of items in a warehouse. The decider combinator sends out an activate signal to the inserters if the warehouse they’re feeding has fewer items than the average. Over time, this should equalize the number of items across all connected warehouses, though you’ll most likely get a bit of constant back-and-forth between warehouses due to stack size bonuses and/or non-integer averages.
You could solve that issue with more combinators to define a threshold (for example, you’d only enable the inserters if the difference between a warehouse & the average was greater than stack size bonus + # of warehouses), but the basic implementation should be good enough 95% of the time.
