Problem Running Multiple Servos - AVR
You do not mention which servos you are using, but it sounds like your 2 combined servos are drawing more current than the computer's USB connection can supply. 

Computer USB ports are notorious for not being able to supply a lot of current.

First you might try plugging into a self-powered USB hub rather than directly into the computer. These can sometimes supply more current than a computer USB port. 

If that doesn't work, next I'd try powering the servos directly from a 5V DC power supply that can provide a couple of amps. 

Report back if any of these fix the problem!