Is it possible to view 2 apps at the same, eg. 2 × Mail, in Split View?
This is not possible in iOS 9. Only one instance of an app can run at any given time.
There is a small change in iOS 10 allowing two instances of Safari to run concurrently, but that is a special case. To do that, long-press a link, and select
Open in Split View.
This can be done only on a newer iPad or iPhone 6 Plus. running iOS 9 or higher. You will notice that these apps are not contained into 1 app, they are each running independently of the other. You can view each app and interact with each app but they can't communicate with each other (other than what's already allowed by the app itself).