I'm a little confused by your question. You are worried about the battery in the air stach for two movies. What about your iPad?
The reason I ask such a question is because it is far easier and quicker to recharge the air stash than the iPad. If you have a way to charge your iPad, you have a way to charge the air stash. You can even keep it on a charger while watching a movie on you iPad. This would work well on a trip where your kid or spouse is watch a movie while you are driving. The air stash can be on charge the entire time.
Yes, you can download a movie from the air stash to the iPad. I am doing one right now and will let you know which apps gets it and how it plays. This feature was added to the app not very long ago. When I first used this feature, it asked me which app was supposed to get the movie. I told it to send it to avplayerhd but it didn't work. Now it does not ask that question it just downloads the movie. So I'm not sure what is going to happen next.
I'm downloading an episode of fringe which is 1.8gb. It takes a while.
Edit: the download finished. It took a long time. It asked to which app to transfer to. I picked avplayerhd. When that app open, it showed a file that was only 143MB, way smaller than the 1.8GB. It would not play.
Then I remembered that the last time I did this I transferred the movie to Goodreader and that worked. So I'm doing it again now to confirm. Iirc, Goodreader uses the built in play on the iPad to play movies. There were two other options in the air stash app to transfer movies too. The default video app is not one of them.