You could do this by compositing a luminance matte that is half black and half white on a diagonal with one of the videos. Place one video on the lowest track, place the second video on the next higher track and place the black and white matte on the top track. Then composite the two top tracks and you should see the videos split on the diagonal. You can make the matte by using generated media. Just add a color gradient and use the default black and white by moving point 1 and 2 very close to each other.

~jr