I have a service manual for the MG-502. The trouble shooting section lists the following as potential causes for low pressure: Partially clogged strainer, stuck regulator valve in selector assembly, broken seal rings on clutch shaft, damaged or worn pump, improper selector valve linkage adjustment. For checking pressure it says to hook a 0-500 psi gage to the 3/18-18 NPTF selector valve inlet from the heat exchanger with a tee. At an input rpm of 1800, pressure should be between 310 and 340. At an input rpm of 600, it should be between 290 and 335.
Temperature range is 140 to 180 degrees F measured at the same port as for the oil pressure check. Max temp at the heat exchanger inlet is listed as 225 F.
The only problem I've had with the transmissions in our boat is when I was messing around near the cable run and disturbed the "free" positioning of one of the cables. That caused issues with shifting (and presumeably impacted the pressures, as it in effect messed up the selector valve linkage adjustment). I knew immediately what I had done, so did not actually check the pressures. If you did any maintenance in the vicinity of the cables, you might look there first.
All of this obviously assumes you have a 502. I'd certainly agree that the nearly concurrent appearance of the problem in both transmissions is a great clue.