People seem to be confused by the Slide Over multitasking (supported on all iPads), and the proper Split View multitasking (only supported on iPad Air 2).
In Slide Over multitasking, the secondary app swipes in to cover 30% of the screen, but the primary app is not resized automatically. Instead, the app is dimmed and behaves the same as if you switched to the secondary app completely on iOS 8 - you can't interact with the primary app while it's dimmed.
In Split View multitasking, you can have two apps side-by-by side in either 50:50 or 70:30 split (there was a rumour of 75:25 split too, but I don't think that made it in). This is proper multitasking mode and the 70:30 split in this mode is completely different to the one in Slide Over - both apps are active and the larger app is resized to 70% of the screen, not partially obscured and dimmed.