Maintaining 5V out from 2 Li-Ion Cells
Try a buck boost regulator like this: -



With any input voltage from 2.7V up to 15V it will produce a regulated 5V on the output. Efficiency depends on load current but it's most efficient range is when delivering currents from about 50mA to about 1 amp with an input voltage range from 2.7V to about 5V.

You have asked for a 5V output and I'm presuming that this will be suitable for your USB devices.