First of all, generally the output should be "linear" in A's and B's. That is, sqrt(A*B) is good but A*B is not, because it gives you 2 powers of A or B. We think like physics and assume these things have units, the output should have the same units.

Secondly, we must be aware of scales. If A & B are scaled in exactly the same way so their numeric values have the same significance, then (A+B) is a good combiner. If they are not scaled the same, then any forms which add A & B are not okay. In that case you only really have one option :

sqrt(A*B) * G(A/B)

Often A & B have the same significance which means the output should be symmetric in swap A <-> B. In that case the G function has limitted forms. I haven't thought about exactly what they can be, but it has to be things like (A/B) + (B/A). In fact if A & B aren't on a similar scale even that form is not okay.

If we assume A & B are on the same scale, then additive forms are okay and it opens up some other options. (A+B)/2 is obviously your first guess.

2AB / (A+B) is an interesting combiner. If an A&B are in [0,1] then this takes (0,x)->0 , (.5,.5) -> .5 and (1,1) -> 1, and sort of penalizes when they're not equal. It takes (x,x) -> x which is a nice property of any combiner when you're trying to make a combiner that can stand in for (A+B)/2.

sqrt(A^2 + B^2) is another, and then you can take simple multiples of these, which gives you forms like (A^2 + B^2)/(2AB).

Anyway the point is that there really aren't very many forms to choose from which satisfy the basic properties and you can easily try them all.

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