Two layers. Scale both and just the position. This is so basic I'm afraid for you. Please go to the After Effects project page and check out the basic getting started tutorials.
There's a LOT of good information on this page about using After Effects. I'd recommend it to you to help get you started.
Also, consider getting some training from one of the many fine Adobe Authorized Training Centers or independent Adobe Certified Instructors around the world. There really is no substitute for in-person instruction for After Effects. Learning this program in a vacuum is very difficult and hideously inefficient.
Hopefully this helps you on your way!
Richard, I know you're quite knowledgeable - but did you really need to add "This is so basic I'm afraid for you?" in your message to what was clearly a beginner in Premier. We should be nicer, don't you think? We all started out knowing nothing, after all.
there are several ways to split a screen. let's look on how to do 2 types. in the examples we are going to use the same video twice, but of course you could implement this technique to 2 different videos which will normally be the case.
say you got this video.
maybe want this result:
or maybe this:
both are not that intuitive to do in Ae. let's look at the first one - the one Rick explained.
1. Duplicate your layer and scale both layers to 50% like this:
2. Move layer 1 to one side, and layer 2 to the other side. if you want to be accurate with your dragging you can add a grid and tick "snap to grid"
now as you drag, the layers will move by exact increments and it's much easier to snap it to the side and not changing the Y value in the process. after that you can add a black solid and un-tick show grid and you got this:
let's look at example number two.
1. Add a grid and also add title safe so we can see the middle of the comp
2. Create a rectangular mask on top of the layer by making the layer selected and then clicking the Rectangle tool and making sure it's in the middle
3. Duplicate the layer, and invert the mask of the other layer. I moved the other layer in in time so that we could see a difference:
4. Get fancy you can add a divider with a shape layer on top of the seam.
I have to say - this is not so easy for beginners, though it seems it should be happy split.