Building new shower, how can I control the flow of water to two different shower heads?
What you want is called a "Diverter Valve".  That will give you something to search for, because you will find all different types depending on they style you want and how many shower heads it needs to handle.  And example can be seen here:

http://www.signaturehardware.com/three-way-in-wall-shower-diverter-lever-handle.html