If you go to www.kohler.com you can select a shower valve and go to "rough-in and operation" instructions. They are detailed instructions and will help you.

Look up Hansgrohe Universal ibox - first of all, you will get a thermostatic valve instead of a pressure balance valve, thermostatic doesn't impact the GPM (gallons per minute) that the pressure balance does. Here is some hansgrohe info. and only will require you to have trim for this universal box - no additional trim needed.
There are various options for configuration but I'm looking on page 16 of the "installation instructions" part No 90111601 and I'm reading "modify the diverter to allow simultaneous use of both functions". This came with my Hansgrohe trim - part number 04231xx0.

I'm obviously a bit of a newbie but hopefully this info helps you understand that what you want can be done. I would go with hansgrohe for any shower fixture because their fixtures are famous for minimizing any restriction on the flow. You of course always want the option of having the maximum flow.

Good luck. Hopefullly the experts will kick in here with more/better details if you need it. I bought my universal Ibox from a plumbing showroom and the salesman was helpful in explaining my options and what I could have done.
Not much more expensive than buying on line.

Here is a link that might be useful: see page 16 of this installation guide for hansgrohe trim 4231

This post was edited by elphaba on Sat, Jul 6, 13 at 21:39

Can this be done easily with three functions with the hansgrohe box or the Grohe system?

The installation info for the Hansgrohe Universal ibox has quite a bit of info in it regarding various configurations you might want for your shower (and of course the fixtures don't have to be by Hansgrohe even though the Univ Ibox is)-
one fixture
two fixture
three fixtures
tub and shower
shower and sprays
tub shower sprays and more
If you want to find out more info on this, I think viewing (there are lots of diagrams) this pamphlet would be a good start. Of course a plumber would be useful too (LOL) but I have read on this forum that some plumbers didn't take the time to read and screwed up the install so always good for the homeowner to be well-informed - don't think anyone would disagree.

Here is a link that might be useful: Universal Plus Hansgrohe Ibox installation instructions

I haven't got my bathroom to the stage of installing the plumbing. I had to change carpenters and thinks have come to a stand still. Its been several months since I thought of these issues. I will be having Hansgrohe fixtures and they are on site waiting for the installation. I will be having a setup similar to Noopd's shower. I will post a link to a thread I started so I could get some specific info. It has pictures of Noopd's shower setup. This setup takes 2 iboxs, and a Quatro rough-in, if I remember correctly. One of the iboxs runs the large shower head that has 2 functions. The other ibox is at the height of the controls. I think it is the Quatro that allows for the operation of 3 outlets and any 2 can run at the same time. It has been a long time since I was sorting this out.

Here is a link that might be useful: Mongoct, Noopd, or Others, Please Advise Me on My Shower Plan