We just put in a Nuheat mat and Schluter Ditra in our masterbath, in the following order, from the wooden subfloor up:

1. Modified thinset
2. Nuheat mat
3. Modified thinset
4. Unmodified thinset
5. Ditra
6. Unmodified thinset (using a 1/4" x 1/4" square notch trowel)
7. Tile

You should check the Schluter site because I believe that installing a heating mat over the Ditra will void the warranty.

That's exactly what I've read on other forums. Warming Systems mats are not different from any other brand mats, so the technology should be the same.
Are tiles heating up OK with so many coats between mats an tiles? That is actually another concern we have. Is any heat lost or it's simply takes longer to heat?

We've only used it a few times in the 6 weeks since the room was done and I haven't kept track of the time it takes to warm up. It does start to get noticably warm within about a half hour, if I recall correctly... possibly less. It definitely gets warm enough to be wonderful and luxurious! We're going with the same system in the guest bath that we've just started to renovate.

To take advantage of Ditra as an uncoupling membrane you're best using it between the heating elements and the tile.

No, not to uncouple it from the heating source. Imho.

I know that the Schluter company asks for the heat to be installed under the membrane, but I would feel comfortable with it the other way too. The closer you get your heat to the tiles, hte better it works.

Uncoupling is for the other mechanical stresses in a building that moves a bit.

We've put Warming Systems + Ditra in multiple rooms in our house. Always wire on the bottom, Ditra in the middle, tile on top. That's actually how Ditra shows it in their installation guide. The tiles heat up just fine.