Simultaneously using both hands - How does that work?
I am not sure I fully understand your question, it's a little unclear. But if I understand the question correctly it is how to we coordinate movement on both sides of the body?

Well this can occur because we have connections between both hemispheres of the brain. The major connection is called the corpus collosum, this is a white matter tract that allows signals from one side of the body to pass to the other side of the body.

In some rare cases there have been individuals who have had this connection severed, for these individuals each side of the body may perform tasks of which there other side of the body is unaware.