The other day, I accidentally found out how to do this.  Anytime you are viewing a message in its own window, just hit Ctrl+Alt+S.  When you do this, a separator bar will appear which you can then place anywhere on the message that you would like it to be split.  Once you have it where you want, just left-click your mouse and your view will now be split.  Each section will show the whole message and allow you to scroll them separately.  If you want to remove the split, just hit Ctrl+Alt+S again and it will be removed.  You can also move the split at any time.

It’s a nifty little feature that I definitely plan to take advantage of.