How should I install a plywood subfloor that will be covered by granite tiles?
Would like to put in a suggestion for either Schluter Ditra or Laticrete Strata-mat.

As uncoupling membranes, they effectively stiffen floors by unitizing the tile layer. For stone installation, Schluter recommends double layer of underlayment.  The second layer should be offset from the first layer, not following seams.  The second layer should only be attached to the first, not through to the framing.   The total thickness only has to be 3/4, but can be thicker as necessary for level matching.  The first layer should be at right angle to the joists, along the the long direction of the ply.

Ditra stone tile install


Ditra plywood install


excerpted from Ditra installation handbook