There could be a more GSAP way to do it, but this can be done by looping it inside of a for loop as opposed to using the repeat attribute:
var currX = 50,
tl = new TimelineMax();
for(var i = 0; i < 4; i++) {
tl.to(".red", 1, {x:currX})
.to(".blue", 1, {x:currX});
currX += 50;
}
See the Pen EPoJBx by Zeaklous (@Zeaklous) on CodePen
This may be the only way due to the fact that the value of the tos cannot be changed on repeat (i.e. new Tweens have to be created).
ZachSaucier's post in Is there a way to translate the x: value a number of times using repeat? was marked as the answer