Possible to charge one phone off another?
Most phones are USB Peripherals (Slaves). They do not provide power on the USB VCC pin. So it won't work at all that way.

Some phones have USB OTG capability. They can provide power over the usb pins, like a host. Some need to be hacked/modded/rooted to do this. Even then, some phones need power injector cables because they can only do OTG data, not provide power.

Mostly, it's very impractical to rely on. Just get a usb portable battery pack.