A simple diverter is an either or device. You need a more sophisticated diverter to do what you want. Maintaining pressure with both on will be dependent on your supply and whether the shower heads are stock or modified...a stock shower head can't use more than 2.5 gpm. Most shower control valves can supply 5-6 gpm max, so it should handle two. It depends on how the rest of the house is plumbed.