At the beginning of the season, let us recall, they devised an underpass which separated the coke-bottle flow along the flanks of the sidepods, feeding the starter-motor slot and the top surface of the diffuser's trailing edge, from the exhaust-flow, sweeping down from the top of the sidepod. That didn't quite provide the type of handling consistency required, and the underpass was duly blocked off.
In Valencia, however, the RB8s have appeared with what amounts to a double-floor. Whereas the first iteration of the idea featured merely a short tunnel, the flow along the flanks of the sidepods now enters a sizeable duct, and re-emerges into the daylight from an outlet between the rear wheels.
There seems to be some sort of separator just inside the duct inlet, so it may be that there are two exits to this duct, and some of the internal flow is directed to the starter-motor slot. A photo from the rear, however, confirms that at least some of this airflow exits under the beam wing, and connects up to the gurney flap on the trailing edge of the diffuser.
So is this a double-diffuser? Well, the point about a double-diffuser, I think, is that the inlet was in the floor, so it increased the mass-flow capacity of the entire underbody, accelerating the air quicker under the leading edge of the floor, increasing the low pressure peak there, as well as accentuating the low pressure peak(s) at the kick-up point(s) of the diffuser(s).
In contrast, because the double-floor inlet on the RB8 is fed by the flow along the flanks of the sidepods, and because that flow is fed by the front-wing wake, it should be sucking the air faster over the front-wing.
The double-floor duct will also presumably increase the flow-rate over the top of the diffuser, and in particular, the flow-rate over the diffuser's gurney-flap, both of which will have the effect of increasing the mass-flow capacity of the diffuser and underbody. Moreover, the stronger the suction through this upper-floor duct, the smaller the quantity of air which gets sucked under the floor just in front of the rear-wheels, a detrimental effect which can increase the turbulence in the outer channels of the diffuser.
So the double-floor should increase both front-wing downforce and underbody downforce. Very neat.