Change this condition
+ player1_MSK: is on ground
to this
+ player1_MSK: [negated] is jumping
+ player1_MSK: [negated] is falling
"is on ground" sets the vars to zero when jumping because is seems that the object is still considered on the ground at the beginning of the jump.
Also I found a typo on the "player2" event sheet, on event 3.
+ System: Else
-> PlayChar2: Set angle to 0
-> System: Disable group "P1 F Right"
-> System: Enable group "P1 F Left"
I'm pretty sure those "P1"s need to be "P2", otherwise it causes the player1 to bounce in the air when jumping over player2.