Installing 2 shower heads
* you're going to want to make sure to balance flow*

In order to do that you will want to loop the pipes around back to itself.




If you want to make the primary loop out of three-quarter inch pipe it certainly wouldn't hurt but is probably not necessary unless you have extremely high flow shower heads. 

Edit

Here's a better image

You can use one or both of the optional valves you can even go with none but then you will always have flow through both shower heads. 

Sorry I tried rotating the image it keeps going upside down 

This is an example of the optional valve[][4)] https://www.google.com/url?sa=i&amp;source=web&amp;cd=&amp;ved=2ahUKEwiOycnFsPPgAhWFw4MKHSSuDd4QjB16BAgBEAM&amp;url=https%3A%2F%2Fwww.qssupplies.co.uk%2FTaps%2FStop-Valve%2FProduct%2F485.htm&amp;psig=AOvVaw0yBhytq_2amzj_lDQGskPk&amp;ust=1552163162650315