Is is possible to produce a resistance that represents the average of two thermistors?
simulate this circuit &ndash; Schematic created using CircuitLab

Use four thermistors. Two of type A, two of type B. Put the A in series. Put the B in series. Put the two strings in parallel. Viola! Total resistance is the average of A and B. 

By type A and type B, I mean type A are thermistors at temperature A, type B are thermistors at temperature B.

You get average of the conductance, not resistance. But it represents the average of the temperatures, within limits.