Verizon could launch them both in the same time period because they have absolutely no stock in the N1. Who cares if they offer a similar phone and people buy it over the N1. They are still going to buy it and still going to use THEIR service. It would make much less sense for them to offer a phone through a 3rd party (google) and then 3 months later offer their own version of a very similar phone.
Aso someone already said with the Pre+ and Pixi+ they are now very similar phones. Pixi is still a little lighter of a version but not by as large of a margain.