Was looking for the exact same thing,
it's indeed not possible to add the timeline from contentWindow, i always get an error, no matter what :
Maybe the contentWindow Mechanism somehow change the signature of the variable.
But i soon realize you can actually control the contentWindow's timeline with it's own functions (like pause() play() and ... progress !).
So i came with this quick and dirty implementation, witch actually works :
var iframe = document.getElementById("timeline1");
var iframe2 = document.getElementById("timeline2");
var timeline = new TimelineMax({id:"masterTimeline"});
var tl1=new TimelineMax({id:"timeline1"});
var tl2=new TimelineMax({id:"timeline2"});
iframe.onload = function() {
var contentWindow=iframe.contentWindow;
var tl=contentWindow.mainTimeline;
tl.pause(0);
// console.log(tl);
// tl1.add(tl);
tl1.to(tl,tl.totalDuration(),{totalProgress:1,ease:Linear.easeNone})
}
iframe2.onload = function() {
var contentWindow=iframe2.contentWindow;
var tl=contentWindow.mainTimeline;
tl.pause(0);
tl2.to(tl,tl.totalDuration(),{totalProgress:1,ease:Linear.easeNone})
}
timeline.add(tl1);
timeline.add(tl2);
GSDevTools.create({animation:timeline,globalSync:false});
Enjoy