@SunsetRunner since your asking for the ability to track with both, i assume you tried and something went wrong. What was it?
I don't see where this option doesn't currently exist, start your house up, then start the mobile tracking up. The only thing is that Fitbit will be getting GPS readings from 2 trackers, and in not sure how the server headphones it with the daily milage.
When I do it I see 2 tracked exents with the same time periods, each event has the milage and map reported by their corresponding device.