Why can't we put the apps we want into multi view?

[Risley]

It would be handy to have the calendar app as part of multi window tasking so if I'm say on the speaker phone in my email I'd like to have the calendar available when planning out business trips. I could probably think of other apps that would be handy but the reality is we should be able to drag and drop any app into the multi view...right?

 

[TheOtherBill]

Not right. Many apps will not work with a resizable movable window. There are ways to put any app in there but some work and some don't.
If the app is truly compatible the developer can mark it as such when he builds it and it will automatically go into the list. If there's one you really need, send an email to the developer and it might just happen on the next update.
For now you can just use the long press on the home button to switch between apps.
There are a few threads on this issue already.