In an interview with the staff from 1UP.com, via the 4 Guys 1UP Podcast (Episode 2/12/2010), Marcus Lehto and Brian Jarrard confirm that there will be no dual wielding in Halo: Reach.

David Ellis: Will there be dual wielding in Reach? There wasn't in ODST, of course there was in 2 and 3...
Marcus Lehto: There will be no dual wielding in Reach, no.

Skip to time mark 24:45 to start the Halo: Reach discussion, and then to time mark 29:39 for the part about dual wielding.
Well, I'm not sure if this is a good thing or a bad thing. I rarely dual-wielded when playing Halo 2 or Halo 3 anyways so it doesn't really affect me.