Alright, so you want to Screen A redirect to Screen B while you can still access to the whole level from both screens?
Then, you will need flags.
First at the screen with the shift (x1003y986), You must make the shift enable the, lets say, the flag 0 with the Flag On option (Circled Red in the attachment).
Then in the screen with the powerup (x1002y986) You click in the Flag A radio and then select Flag 0 in the drop down list (Circled green) and then you, in your case, must input in Target Y (Circled yellow) 985 or in Map Y (Also Yellow) -1. So the "Targets" end up something like (x1002y985).
Then, if you want to enable (x1002y986) again you just have to put Flag 0 in the Flag Off option (Circled Blue).
I hope I make relatively clear, also remember that sometimes the best its to Trial and Error yourself.