A two way valve will run two outlets for example an over head shower and a side shower or an overhead shower and a bath filler. The valve will have at least two handles, if it has two handles one will be a diverter and one a temperature control and if it has three handles there will be two independent on offs and one will be temperature control. These may be called twin or triple valves, depending on the amount of handles. You will have a choice of valve design and handle design. Each one will have a minimum recess and minimum pressure requirement, your plumber will be able to advise on what is going to be suitable for your install.