Is it recommended that 1/8 thick plywood be used as underlayment?
If the subfloor is plywood no other ply is necessary; but you do need a flat surface for the Ditra.  If the subfloor cannot either be scraped, chiseled, or sanded flat then self leveling compound may be required. If you use self leveling compound remember that the surface should be primed first - follow the product's prep recommendations.   1/8" plywood, in my opinion is not sufficient; unless it was set down on flat surface with troweled adhesive and stapled 4" o.c. I would recommend 1/4" min - which can be found at most flooring supply stores - specially formulated for flooring; i.e., all voids filled in the plys and water resistant glue.