I've got a Colt AR-15 SP-1 and I think I've got an assembly manual for the A2.  Once I get the take down pin out of the way, it tells me to pivot upper receiver from lower receiver, which I did, then push receiver pivot pin.  The only thing I see in the same vicinity as on the diagram is two flat-head screws that connect to each other somehow in the middle.  I'm not sure where to go from here.

All you have to do is unscrew them from each other. Everything else is the same. Who the dumbsh*t is that came up with that idiotic screw is, noone knows, but I hope that his old lady cuts him off every night, and his dog pisses on his leg every morning on his way to the car, the stupid m*therf*cker.

For normal cleaning, it is not necessary to separate the receiver halves.

If you want them separated, just unscrew the screw on the safety lever side of the receiver, then push the remaining half of the pin out from the safety side of the receiver toward the mag release button side of the receiver.

Use two screw drivers, one on each side.