I have an upstairs shower that I want to add a second shower head to. The second head will be on the opposite side of the wall. The current shower head puts out 2.5 gallons per minute I think. If I buy two new shower heads that have a lower output rate will that allow me to have enough pressure in both heads?

We can't answer this since we know nothing about the size of your supply pipes. If you tap into your supply with a new mixng valve tapped prior to your existing valve the volume will drop but not necessarily in half. Do you know anything about the pipe sizing?

The flow is limited by the head to a MAX of 2.5. Don't know what pressure you have but putting in but 2 smaller flow heads would most likely work. You can get them down to 0.5 GPM. Google :Low Flow Shower Heads for reviews.

I believe it is .5 in.

Ok. Thanks

1/2" isn't going to work. If you want this to work you can use 1/2" but it must be tapped into the nearest 3/4" supply which should not be too far away. Most baths are plumber with 3/4 and then 1/2" taps go off to each fixture, just make sure your new fixture has its own home run back to a 3/4 and you should be good to go