I switched to Kaiten EMail app. It is similar to K-9 but set up for tablets much better. You can enable and disable the split screen in the settings and also adjust the size of the reading pane compared to the list on the left when you do have split screen on.
Actually, Kaiten Email does no better on the tablet than K-9 mail and I refuse to pay 4 bucks for an app that was ripped off from a free open source app.