Drag them to iTunes again when connected to the iPad, however it is possible there could be a better official happier way. I hope you don't need 2 files of the same movie, it wont make to much sense.
The iPhone videos should work fine on the iPad as the top limit on the iPad video capability is higher than on the iPhone, even than on the new iPhone 4. You could find out exactly on this post. Screen resolution / image quality on iDevices
I mean the iPad video player is way more capable than on any iPhone but the encoder file format is the same H.264. I don't believe there is a problem such as too small video for the iPad. Video capabilities are described as "up to this size". In the end if it plays on the iPhone it should play fine on the iPad. Now your problem could be How to sync the same videos on 2 different devices using the same computer and account.