I have iPad 2 and updated iOS 9 but still could not use that double window option , as of before update I have see it was writen that this option can be use on any device of iPad
iPad 2, iOS 9.0.1
My understanding is that iPad Mini 2 & 3 can not use all of the functionality of iOS 9, the split screen apps is one of those features not supported by the hardware. Mini 2 & 3 have 1GB RAM, the Mini 4 has 2 GB and can use all of the new features of iOS 9.
That option can only be used on newer iPads. Older iPads such as yours do not have the necessary processing power to support the feature.
I have iPad 2 and updated iOS 9 but still could not use that double window option , as of before update I have see it was writen that this option can be use on any device of iPad
