Charles Wood , Sep 05, 2012; 09:54 p.m.
I would suggest a separate microphone system but it depends entirely on what type of video production you're recording. For a talking head/interview situation, an interviewer with a hand mic can work. If you're recording multiple individuals such as a stage production, not only would you require multiple mics, likely wireless, you'd need to have a minimum of a two camera shoot, mixing board and feeds to both DSLRs.
Not to rain on your parade but creating high quality video productions with DSLRs requires a lot of additional gear. If you have a look at http://www.cinema5d.com/ you'll quickly get a far better overview of what is required than you're likely to find on this site.