I have had a Fi phone for a few weeks, and it seems pretty agnostic between the two networks - it just takes the one with the strongest signal at any given point in time. I believe under the hood it is connected to both all the time, and selecting which one to use in software (and reevaluating this decision on the order of seconds).
The Google MVNO probably just has roaming contracts with both - this sort of thing is part of the GSM standard. In the UK Three and EE apparently share their networks, even though they are two completely separate companies [0].
Except Sprint is CDMA, and T-Mobile is GSM - again, for data its easy to do, both networks can use the same APN, so handoff is seamless and transparent, for voice, its less so.
That's easy to do with data, both use the same google APN end point, voice its not so easy to do, even with selective call forwarding, its still less then elegant.
