Check out this video tip from Apple on working with multiple emails at a time on iPad. This is a new feature to me, and I bet for a lot of other people, too. The short version is that you can use the handle at the top of an email you’re writing to drag it down to the bottom of your screen, where it stays docked. That allows you to peruse your inbox. If you have just one docked email, tapping it brings it back up. If you have more than one, it gives you a Safari tab-like view to pick the one you want. It’s a nifty feature. Oh, and it works in portrait and landscape mode.
Check It Out: Quick Apple Video Tip on Managing Multiple Emails on iPad
That feature has there been for quite a long time. as far as I know.