Anyone done double coat/ double exposure / double development ?
Yes. But the nicest ones were cynotype over pt/pd and gum bichromate over pt/pd. Double coating pt/pd on any cotton paper never really made much of a difference for me. As mentioned, double coating on fixed-out baryta makes a huge difference (and is in fact, the only way to go).

For my cyanotype and gum over pt/pd prints I used cotton papers (Platine, Cranes cover and others) with the old "pin through the edges of the neg and paper" trick to register everything. But before I applied the first coat, I soaked the paper for about 15-30 mins and then let it dry. That, in my experience, took care of any issues of dimensional stability between coats and the registration was fine.

FWIW, both the cyanotype and gum second coatings really added a nice extra bit of "umph" to the prints. But the gum bichromate prints have a very nice extra bit of sheen that gave the shadows some more depth.

Here is a lo res photo of one of the cyanotype over pt/pd images. Forget about seeing any of the subtle tones or depth in such a poor reproduction, but you can see that the registration is decent.