Building a new master shower with antique-looking exposed copper plumbing

Hi, I am building my own house and roughing in my master shower. I want dual shower heads with an antique, industrial look with exposed copper shower plumbing (see diagram). Initially I ran 1/2" pex, but I was told I would need more volume for a dual shower, so I ran another set of 1/2" pex (it was easier than running 3/4 since I already had a 3/4"-to-1/2" manifold). So now I have two 1/2" pex hot lines and two 1/2" pex cold lines that I need to connect back together with a pressure balancing valve (per code). Would either of the ideas in the diagram work? Which is better? Is there a better idea I'm not thinking of?

Since the cold water flow volume will be so much lower than the hot water volume, adding the second line for the cold side wasn't really necessary, just adds to the complexity.

Idea #1 works, #2 may encounter some unanticipated interations/oscillations at some flow rate.

In any high-flow shower there is usually payback on installing a drainwater heat recovery heat exchanger, since it allows you to down-size the hot water heater, and will cut energy use for showering loads by about half, provided you get a decent sized one (4" x 48" or, 3" x 60" or taller.)

One 3/4" PB valve would likely be more reliable (and less expensive) than two 1/2" ones in series/parallel. I'd consider running the two 1/2" pex lines into a 1/2x1/2x3/4" T, and feed the single valve with that 3/4" outlet.

I was having trouble finding a 3/4 PB valve - plumbing supply houses hardly knew what I was talking about. Do you know where I could order one? Thanks!

I have only seen 1/2" PB valves, and I will venture that you have no idea how small the ports are in that type of valve. It would probably take at least 3 of them to equal the area of a 1/2" line, (well maybe only two for a 1/2" PEX line).