How do I handle a VirtualHost/Passenger entry for a Rails site that solely handles subdomains?

Question

I want to use Rails 3's new subdomain features to handle all subdomain paths except www and nil.

Oppositely in a second rails app, I want to solely handle www and nil in the url path.

What would my two VirtualHost entries look like for Apache with Passenger? And in the code, for the first subdomain handling app, so long as I handle the new exposed :subdomain constraint in routes properly to ignore www and nil, that should prevent any conflict right?

Thanks for any input.