I want to power my Arduino from my computer because I need to talk to it. I don't want an extra power supply, if possible, to power my motors at 5 V and ~0.5 A (total), but I believe that's too much for the Arduino 5 V pin. Can I use a USB splitter to have one USB-A cable power my Arduino through the micro-USB port and also the motors via a second micro-USB end?
You COULD, but I woulden't as you way distroy the computer!
500 mA is the MAXIMUM for single USB port (exc USB 3.0)
As MissDrew states you could kill something on the computer using your method.
Suggest you read up a little on USB specifications.
I could have sworn it was 2 amps. Thanks for the heads-up.