If they are running by two separate programs then yes.
On Android devices if you long press (hold it for a second) on the home key it pops up a recent list of the last 6(<=2.1) or 8(2.2>) programs you have ran. It is very easy to simply press the home button and select the other program. I have used it to easily change from a pdf file to a book I was reading.
Since the Nook does not have a home key I am sure there are easier ways to replicate this button and I hope others post some ways to do it here.
As of now I simply put an app shortcut to SoftKeys on the screen so when I press it it pops up a recent list. This can be turned on under settings.
Easy really.
As far as I know I know there is no way to have the same program display two different documents back and forth. I am sure it could be done but it would have to be written into the program. I would just use two programs.