The split-screen feature is demoed in the iOS simulator on a Mac. It shows how the size of the currently running app can be resized to 1/2, 1/4 and 3/4 the size using two-finger gestures. It is not clear from the video how to open the second app next to the already opened app, but that could be probably because the feature is still being developed.
The code for running side-by-side apps was discovered within the inaugural iOS 8 beta, again by Troughton-Smith, a few days ago. The software update is expected to be released to the public sometime this fall, but there’s no word on if this feature will be ready in time.