What tile are you installing.....the ditra will cope with lateral movement ( expansion of the timber floor) no problem.....
Now i believe with your terminolgy ( lol ) that your are usa based.....
so you have different standards with ditra to the uk.......
You will need to stick the ditra down with modified adhesive and then tile on top with standard set...but i do belive you can get Ditra set in the usa...if location is wrong then i will tell you the uk way...
As for the subfloor then you need to make sure it is well screwed down and no movement......always follow the adhesive manufactures guidelines of subfloor preperation for tiling ( i.e. priming )........