Import video from the ipHone to your computer.
Then import the video to the iPad via the Photos Tab in iTunes.
The video will then be in an album on the iPad, and iMovie iOS will be able to see it.
Alternativly, you can import the videos quickly into a project on iMovie on the iPhone.
Choose to export the project to itunes.
Go to the apps tab in iTunes.
Scroll down to the File Sharing part.
Open the iMovie App, and copy the project out to your computer.
Then plug the iPad in and drag the project into iMovie on the iPad.
Then import the project in iMovie on the iPad.
You can use the camera connection kit from Apple to directly transfer video and Photos too; but it costs.
There is more information in the iMovie Help files:
http://help.apple.com/imovie/ipad/1.3/index.html