Servo and power supply problems
The problem isn't that you only have one 5v pin, the problem is that you are exceeding the half-ampere current limitation of the USB specification, and causing some sort of protective circuit in your PC to activate - fortunately that is happening, otherwise you would damage the port.
Something like a phone &quot;charger&quot; used on its own may give you a bit more power as these are often willing to supply an ampere or two to a phone that figures out it is plugged into a charger and not a computer, and so draws more to charge its battery faster.

Powering both servos from an external 9v battery.

That's doubly problematic, first because it is will outside the allowed voltage range, but next because while the designed voltage into a light load is high, the available current is far too low to properly operate a servo.
You need a high current 5v supply, not a high voltage low current supply.