two shower faucets in one water supply line
Larger supply piping and a "parallel" (as opposed to "series") manifold arrangement will help, but you need to realize that there is only so much you can do. You are limited by your home's water supply pressure and size.

Probably the easiest thing to do is use low flow "water saving" shower heads/nozzles.  

If you want to do more, check the size of your: water meter, water meter union, and main water line. Increase the size of all if possible. Also, check to see if your main water supply has a pressure regulator installed. Check your static (no-flow) pressure downstream of the regulator to determine if you can raise the regulated pressure (check your city bldg. office for max allowable pressure). Older homes with steel water piping often suffer from low gpm/lpm flow rates, consider a complete re-pipe if this is the case.