Ipad 12.9 first gen behaves exactly like all 2GB ram devices (9.7 pro, mini 4, etc.) by freezing the split when the slide over is active, this while having 4GB RAM... 3GB devices support 3 at same time...I can confirm that the 1st gen 12.9 does support this configuration, but the split view apps are 'frozen' when the slide over app is active - I'm not sure if this is the case for all iPads? (this is in iOS 12)
Sometimes I wonder what those 4GB RAM are there for? Does it refresh less often? Maybe... But my mini 5 seems to hold apps in RAM better than the 12.9....I haven't done serious tests for refreshes, it's just my impression so far.... Hopefully the IpadOS will improve things...