Determine which style of dual shower head you want. Options include ones with side-by-side shower heads, one stationary and one on a flexible extension, and one stationary one with the other on an extendable arm.
Remove the existing shower head from the faucet by twisting it counterclockwise. If the shower head is on tight, you might need a pipe wrench to remove it. Wrap a piece of cardboard around the head first to protect the finish from scratches before using the wrench.
Screw in the new dual-head fixture clockwise onto the open pipe by hand. If the joint leaks when the water is turned on, then tighten the connection slightly with the pipe wrench. Do not over-tighten by forcing the fixture onto the pipe.
Adjust the shower heads to cover two people in the shower at the same time.