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.