<shrug /> I can't see Nokia releasing a device big enough to hold two batteries; an extended back allows bragging about slimness and pocketability, while also hitting range (with optional high-capacity battery).

And I see suspend on back-cover open + backup capacitor (HP48) or backup coin cell (HP50) as a perfectly viable hot-swap method, which should be implemented.

But it's 2 vs. 1 now, so I yield: two batteries, with an extended back allowing two high-capacity batteries.