Step 1: Materials, I used 1/2 in tubing and fittings. 3, 90 degree fittings, 1, 45 degree fitting, 2 threaded fixtures for the shower heads. 1 pack of pipe straps, 1 divirter, 1 pipe, 1 shower head, plumbers tape and some pvc glue. I'm pretty sure you could use thicker tubing to allow more water to flow but you will have to figure out how to connect to the shower heads. The total cost on this is about $35
Step 2: assemble all of you parts together in the hardware store so you don't make any mistakes.
Step 3: Bring home your stuff. Assemble the double shower up to where it will turn 90 and go across the shower. attach this to the wall using pipe straps or screws and cable ties. Don't glue any thing until the whole system is test , just in case you need to make adjustments.
Step 4: assemble the divirter to the existing side. Now you can take a measurement between the divirter and the wall. Cut a short piece of pipe that will reach the wall. Add a 90 on the end and then take a measurement from one side to the other side of the shower. Cut a longer piece and fit it in place.
Step 5: now while hold the pipes together turn on the water to check flow and pressure. if adjustments are needed, do it up. Once it seems to work properly take it apart and glue and tape all the connections.
Step 6: Take the most awesome shower ever, then have a really good day.