Per MacRumors iOS 12 page...
But on my 9.7 iPad Pro I still get the same “non usable” behavior when doing this as I got on iOS 11....
On iPads with 2GB RAM, up to three apps can be used at once in iOS 12 by pairing two apps in Split View with one Slide Over window. When attempting this in iOS 11, the two Split View apps were not usable while a Slide Over window was open.
But on my 9.7 iPad Pro I still get the same “non usable” behavior when doing this as I got on iOS 11....