Android N is finally making multi-window a reality in stock Android, and the way it works is already more slick than what Samsung has been doing. Since not everyone has a spare device on which to install the developer preview, here's a quick look at how split-screen mode works.
The device used for this demo is a Nexus 6—it's big enough that split-screen is actually kind of useful. Most apps show a toast message when opened in split-screen that they might not work. That's just because they don't target Android N, but everything has been fine so far; just a few UI glitches. You can even launch other apps from split-screen and they will stay in their half of the screen.
Some of Google's screenshots show buttons on cards to go into split-screen, but on the N6 all I have are the long-press gestures. You can long-press and drag a card from the recent app list to the top to split screen it, then choose another card for the bottom. Alternatively, a long-press on the overview button opens a split screen with the current app. If you turn on the split-screen swipe up in UI tuner, you can drag up from overview to set the location of the divider as you simultaneously open multi-window—that's what I'm doing in the video above. We're still checking out Android N on tablets, so there might be some differences to cover later.