We use step-up converters built into the ethernet socket to power up PoE enabled 5GHz WiFi gear using power from laptops' USB. This can be used for in-field signal testing. We use it mostly with Mikrotik and Ubiquiti devices, which need only 8V to function properly, so we use step-up to convert 5V from USB to suitable 9V. In all cases i've tested, single USB port was enough to power all tested devices (even when my calculations didn't said so). However you can still add second USB cable to get another 500mA from another USB port…

• LM2577S step-up converter module
  • pot needs to be adjusted to 9V before use
  • http://dx.com/p/fc-26-lm2577s-adj-dc-dc-power-supply-step-up-module-for-arduino-179398
  • http://jv-shop.eu/index.php?route=product/product&path=59&product_id=73
  • http://dx.com/p/produino-dc-3-5-24v-to-dc-4-0-30v-voltage-step-up-boost-module-red-282902

I simply fit the step-up converter into wall-mount ethernet socket and wired everything up inside as PoE injector.