Set the starttime of a video layer

In premiere pro, I have a lot of video one after the other. I want to make a transition between each of them (the same transition each time). Th transition is quite complex for Premiere, so I want to create a mogrt in after effects.

So in AE, I have two video layer. But I need to be able to change the start time of each one.

In PP :

-video1-mogrt-video2

in AE

-end of video1-start of video2-

in mogrt

cursor 1 : time to start video 1

cursor 2 : time to start video 2

In PP, I will change the videos and the cursors in the mogrt, each time I'll use it.

But in AE, how can I change the startTime of each videos at the "start" of the mogrt ? Like this kind of script :

on startmogrt() --execute this just before playing the mogrt

  video1.startTime=cursor(1);

  video2.startTime=cursor(2);

end

I try to put this (correct) script in lot of place (nul object->position, remap time of  video layer, ...) but, of course, nothing works !

My question : is there a place where I can write a script that is executed at the start of a mogrt ?