Many of the manufacturers suggest running multiple showerheads via a loop rather than single ended. That way, the pressure will be the same at both (all). Two heads shouldn't be more than 5 gpm. Most valves can supply that unless your feed lines are old and corroded (like galvanized lines). You might pick up a pressure gauge...around $10 that you can screw onto a hose bib to check your static pressure. Unless you are one a well, it is probably okay.