You guys don't have it?
Hit the recent apps button then when the apps come up hit the icon at the top of whichever you want, split screen is in the menu that pops up.
1) Click on the Menu that shows all open windows. Each slide has a round icon for the app at the top
2) Click on one of those round icons, there will be a short list of options, and one is 'Open in Split view'
3) Once one of them is open, the next will of course open in split view
I don't know what version you have, so I am not sure what your button looks like to open up to see all active windows, mine was to the left of the home button on my last phone, the one I have now is on the right. Good luck!
EVERYONE- This is much better:
Download GOOD LOCK.
An app madeby SAMSUNG to replace all the cool features that Android took away. It reactivates split screen view & One-hand-operation features, AND Pop up view back to how it used to be where you just swipe down from the corner, or hold recents button. Makes it work like the old Samsung 7/Note 8 used to. but it doesn't change anything else with your phone whatsoever. The only thing it does is allow you to swipe down from corner for pop-up view, or hold down for recents for split view, or swipe up for one-handed operation. It also turns all apps on to be able to open up in pop-up view. right now Andrew wants you to go through one by one to try to turn them all on but this app does all for you automatically. It's a setting so you can tell it not to do that if you don't want it but there's absolutely no reason not to.