IOS 9 multitasking works on the ipad air 2 right now. There is no forward statement to make because the current ipad 2 has more RAM and can support the feature. It is a given that this will be in the next generation ipads as well. The iphones are more of a question at this point.
https://www.apple.com/ios/ios9-preview/#ipad
Yea the iPad Air 2 has it I know because I'm using the public beta, I'm not so sure about the phones it depends if Apple wants to do that with the phones. I love the multitasking on the iPad but I do think it would be even better on a bigger iPad, this is where I think the iPad Pro comes in. As for the phones I'm not sure if it would be good to have on them, I won't say no because I could be convinced but I won't say yes because a part of me thinks it wouldn't be as good as on the ipad, even with the size of the 6 Plus I'm using.
 will render the iPhone too small for Split View, I'm quite curious. Ever noticed that reachability on the 6 Plus takes exactly half of the screen? Maybe instead of just showing the top half of an app, they can just optimise the apps, and make it a true one handed mode like Sammy are doing. And then Force Touching on the top half can bring up another app. This could work.