Ive seen a number of trucks in here with compound setups, and always had wet dreams about one on a rotary.
I know there's a kit for supra's using a compound plumbing setup, and its laying down RIDICULOUS spool and insane top end on the 2JZ. Search around the net, can't recall the name of the kit ATM.
Personally, I think taking the general idea and adjusting it to suit the rotary's strengths is the best approach. I would personally like to see the small turbo fed first, but reversing the direction of the intake air. Have the large turbo suck through the smaller one. Thoughts behind that being: the smaller turbo would have -essentially- vaccum on the compressor which is going to help it gain shaft speed very early. These babies like the air volume as we all know
And once that snowball effect starts to roll on as the exhaust pumps out more and more pressure and volume - well we're off the races
I do feel though that cooling will become a larger issue with this setup. We know its a problem that you have manage well on these things already, and 2X the exhaust plumbing, 2X the compressors etc is going to create a larger easy bake oven then a single turbo. It may be worth considering a small cooler between the first and second turbo (which ever you mount first) like one of those tubular style babies. Just something to add some extra density and take some load off the main intercooler.
I love the idea, and think that with a well tohught out setup, we can see a true example of what Mazda may have designed if the intended the FD to be 550whp from the show room
If there was ever a man I would have hoped to tackle this task - its you Brian. If you head down this road, I know its going to be all it can be.