Hello Jim,
One pressure switch is for reverse, the other is for forward. They complete the ground (earth) to the idle speed solenoid on the intake linkage and increase the idle speed when the car is in forward or reverse.
They are the source of the negative ground (earth) since they are bolted to the transmission. Yes they are wired in parallel.
The function changed on the later USA cars, a dash pot was added to the intake to increase idle speed in gear. The linkage solenoid was then used to increase idle speed when the AC was turned on.
As far as the wiring on the pressure switches, they are tied in parallel with one wire between the two and then onto the terminal block of the transmission with another "grun" & "rot" wire that goes to the intake linkage solenoid. It should be one of the wires with a 4.3mm inside diameter ring terminal hole . There are three different ring terminal hole sizes.
Power to the linkage solenoid comes from the fuse box #5 via a "swartz"/"grun"/ "rot" wire.