So, if both locos are set as ID1, so that both will pick up the track signal from the single remote commander, they will then act in unison: Correct?
Correct
Also, is there a way to make on of these locos run backwards, like I was able to do with my second Lionel Conventional Grand Central express FT lash up, but reversing the two wire plug inside the engine?
You could reverse the polarity of the motor leads to run an engine as you suggested. You will will have to swap the yellow and white wires on the power / motor lead plug that goes to the PS2 / PS3 board.
And: Can a PS2 run in a lash up with a PS3?
Yes it sure can!