What are the effects of implanting electrodes into the brains of monkeys? If one implants electrodes into a monkey's brain in order to allow the monkey to control a mechanical arm, then is the monkeys able to move both it's natural and it's mechanical arms at once?

What happens if, say, the same signal might tell the mechanical arm to move up, and the natural arm to move down?
The interface between the brain and the hand is not like a USB computer port, and we don't unplug the biological arm and plug in a mechanical one instead. We implant electrodes to just a tiny fraction of the neurons in M1 (about a hundred, out of millions), and use their signals to control the mechanical arm.

So the answer is yes. The monkeys don't lose any dexterity in their biological arm because of the Brain-machine interface (BMI). Actually, during the BMI experiments, they usually move their biological arms all over the place. There is no reason why, after enough training, they couldn't control the mechanical arm almost completely independently of the biological arm and at the same time, like you control your digits (almost) independently.