I have a new Fox 36 with the all new Float air system: no more coil spring for the negative spring system.
You can find all the technical detail here:
2015 Fox 36 FLOAT Tech Unveiled ? All New, All Air System

BUT, for my ECDM 27.5" tandem I have change the travel to 120mm (via Fox spacer, it's an option). And I was disappointed because even when you inflate the fork to the 125PSI max pressure, as the negative air spring is automatic as soon as the 2 of us are seat on the tandem with have a sag of 50% of the travel: really to much.
Because of the negative air chamber volume you could even inflate the fork to 300PSI it will be the same at the BEGINNING of travel.

THE SOLUTION: reduce the volume of the negative air chamber. Doing so the negative air spring will stop acting closer to the beginning of the travel stroke.
I've done that simply but putting an old fork end travel bumper around the main piston tube letting at least on of the small air port open:
2014-2015 Fox 36 Float negative air spring setting for tandem-p1050646.jpg2014-2015 Fox 36 Float negative air spring setting for tandem-p1050647.jpg
And it's just working !!!