On the one hand, maintaining two OS teams for the same hardware would be a waste of resources. On the other hand, I have heard stories of how Microsoft would often commit several teams to build a product to the same spec: the team that got something market-ready first was handsomely rewarded, and the teams that didn’t got reassigned to wash Bill Gates’s car. On the other other hand, Microsoft (alas) has more cash to burn on this kind of adventure than Nokia does.
Nokia’s pre-Elop strategy was to encourage app developers to use Qt, which could be compiled to run on either Symbian or Meego, and then the company could gracefully phase out Symbian in favor of Meego. I work on back-end stuff rather than on apps, so I don’t know how well this could have worked in practice, but I did observe that when my company phone was a (Maemo) N900, the Ovi Maps client for Maemo was several revisions behind the client for Symbian.