Will 2 people be showering, steaming and sitting on benches at the same time on a regular basis? If so (and it's not that common), then 2 benches will be fine.
However, I will tell you that DS has a built-in bench in her steamer and never uses it without putting a towel down because it's hard and cold, even with the steam. I like freestanding teak benches instead. But that is a very personal choice and people have definite opinions on that issue.
If you decide you want 2 benches and thus need to move the door to the center, you can solve the valve placement issue by putting the valves in the pony wall that looks like is already drawn on your diagram. If you will have a pony wall on either side of the door, you can choose which pony wall to put the valves in. Make sure the door swing is flipped around opposite of my picture if you put the valves in the upper right pony wall.
You might want to make one of those showerheads a hand-held (it can be on a slide bar) for ease of cleaning. If doing that, put it on the one closest to the bottom of your picture, and place an extra mount for it lower and by the bench so that whoever is sitting there shavings legs can put the handheld on the extra mount. Alternatively, you could put a 3rd fixture, a hand-held set low and by one of the benches for shaving legs; that's what we have, you just need to get a valve that will flow to all 3 fixtures.
Regarding the door. Will you have a transom above the door that you will tilt to leave open for evacuating the steam? We do, but some people don't like them. We didn't want the door to be so heavy, so we put in the transom. Either way, I find it convenient to allow the door to swing inward and outward. The glass person told me they just don't put the stops on the the hinges. One glass person told me we would wouldn't get as good of a seal, but we haven't noticed that at all. The reason I find it convenient to let the door open inward and outward is because after the shower, even with the transom open, we like to leave the door pushed inward a bit to help evacuate the steam better. I didn't want it sticking out into the room while we are both getting ready, so we leave it pushed in.
Good luck!
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Steam shower design
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