Arnea is right. This is a modified IFP filling valve. It is more like a tuning/maintenance thing. Not at all related with the SPV stuff.My old Manitou Swinger 3 way SPV had 2 air valves as well. SPV was Manitous pro pedal and it is there to help reduce the heavy pedaling feedback.
It is older and i found this regarding the SPV, but i don't really know what it means "The pressure on the damper in the chamber affects the compresion. The ramp up not only affects a small air spring, but the damper too, so you can adjust end stage damper."
I remember that i needed to get the air pressure correct in both chambers for it work properly, if not, the rear end had issues (either too much bob, or too stiff. However, when i did get the pressure correct it worked quite well.