You'll want to solder things, forget the glue.
A divertor valve may end up less expensive than two control valves.
Since benches are often a source of leaks and compromise the integrity of a shower, I suggest you look at a couple of products: Kerdi from www.schluter.com. It will make building the shower and bench much easier and ensure a nice, waterproof, long-lasting shower. A monument (solid) bench is one way to do this, but another is the Better Bench from www.innoviscorp.com. I've personally used both, and like them.