Why not just put a T (facing up) on the outlet with a ball valve and run plumbing back to both tanks with ball valves on both of those tank inlets. Then you just have to turn a few valves, and you can move all your fresh water into the salt mixing tank, or if you just want to circulate your RO/DI water (don't know why you'd need to, but it would make the piping look symmetric, and I like symmetry), so then there's no flex tubing or anything, it's all plumbed with rigid pvc.
Also, on the "water level viewer", make marks so you know that XX level is XX gallons, I guess by filling first time with a 5 gallon bucket or something, making it easier for measuring salt.
Also, if you have the wiring skills, why not put the powerheads and pump on 3 switches on the stand or whatever you're planning to put this on. And if you're planning to go straight from the mixing tank to the display tank or sump or whatever, you should probably throw a heater in there as well so it maintains a constant temp.