Mixing different digital cameras in the same movie
HD 1080 video is only 2 megapixels, so the camera full size rating would not be an issue for video.  You have to put all of the video clips together in a video editor of course, which should not any issue.  I would shoot with both cameras at the same settings, like 30p. 

There is always a possibly of slight color/tint differences in two cameras.  Probably not noticeable on different scenes, but different views of the same scene possibly might notice? The video editor should allow correction in some degree even it does show.