I like the idea of the overflow drawing a large proportion of its water from the bottom, but this set-up does indeed seem like one that is intended to look impressive and high-tech, at the expense of actual utility. I can't help but wonder if, in a planted freshwater tank, there won't be a considerable amount of vegetable matter tending to get caught in that awkwardly placed gap and causing clogs.
Since the end product will be nicely concealed behind your internal background, how about this: Remove and ditch both inner and outer weirs, and instead simply install a vertical standpipe into the bottom bulkhead. Extend the pipe to the surface, and at the top install a 180-degree U, then another vertical pipe reaching back down to near the bottom. Drilling a single small hole in the top of the U, above the waterline, will allow this arrangement to function as a simple overflow rather than a siphon. In this configuration, the overflow will be drawing entirely from the bottom; the end of the pipe will of course be fitted with a screen or sieve to prevent the entry of fish and large plant fragments. An enhancement would be the addition of a water-level intake at the top near the U, ideally set up in a manner that would allow you to change or adjust the size of the upper intake hole to therefore control how much of the water is drawn from the top, with the remainder coming from the bottom of the tank. Ideally, you would install a shut-off valve just below the bottom of the tank, so that if for some reason you need to remove the entire gizmo for service or alterations you can ...you know...not drain the whole dang tank.
Speaking as a cheapskate DIY-er, I can tell you that I have used this set-up on numerous homemade and commercial tanks of all sizes, and it works wonderfully. It is butt-ugly if left visible, but it matches or exceeds the functionality of commercial look-cool weirs and is easily changed or modified to achieve the balance required. It can also be adapted to use with top-drilled tanks simply by attaching a 90-degree elbow at the side bulkhead, and then extending a vertical pipe to the bottom, with all other details the same as the bottom-bulkhead type. If used with a top-drilled set-up, it really calls for a U-shaped extension on the outside of the bulkhead, installed with a union that allows you to rotate the external U to fine-tune the water level in the tank.
Yeah, I know...I'm cheap...
