iPad 2, 3, 4, 1st gen iPad Mini ( which is functionally the equivalent of the older iPad 2), iPad Mini 2 and 3 and iPad Air ALL have 3, 4 and 5 year old hardware and RAM limitations that prevent ALL of those models from being unable to support split screen, and all but the iPad Air, Mini 2 and Mini 3 can't support the Slide Over feature, either.
As of iOS 8, older iPad model/users have only been getting the basic, barebones features of each newer iOS version.
Mobile device technology moves at a much faster pace than traditional computer technology.
The much older hardware in older iDevices is, simply, not up to the task and these features would simply slowdown/cripple the older iPads' performance even further.
Apple is DONE with iOS 9.
Older iPads will NOT be getting iOS 10 OR the more advanced features of iOS 9.
Not going to happen!
If you want all of the latest and greatest new and cool iOS features, you will have to consider purchasing the newer iPad Air 2 or iPad Pro or an iPad Mini 4.
Good Luck to you!