What is the minimum spacing between inductors to prevent cross talk?
A coil will produce a field (Bx) at some distance along the centre axis like this: -



Where 



n = number of turns, I = current and x and r are dimensions of the coil and spacings.

So how much field can you suffer with cross coupling. Clearly you will get some at any distance but, beyond a point (x >> r) the field falls as distance cubed.

It's also a reasonable assumption to make that at those sorts of distances, the field received will tend to be constant in the plane of the receiving coil therefore you can calculate total flux entering the coil and, using the frequency of your signals calculate rate of change of flux.

E (induced voltage) = N\$\dfrac{d\Phi}{dt}\$ where N = number of turns of receive coil.

So, there are formulas and little rules of thumb. How much induced voltage you can suffer is beyond my knowledge.