The cell phone number isn't in the phone but on the sim-card. So you need to put two sim-cards (for 2 different numbers) in the same phone. With the search terms in the subject line google finds some possibilities, both special phones as dual sim holders for in any normal phone.
I've got no personal experience with any of them.
But you aren't required to have Nextel/Sprint service.
I know it's out there because I saw an email advertising it but I deleted it and can not for the LIFE of me find it again. Anyone seen or know of what I am talking about?