I like the set-up shown by
duanes
^ here. I'm not a fan of bottom drillled tanks with standpipes, simply because it is possible...don't ask how I know!...to damage the standpipe or its bulkhead and have the entire volume of the tank drain out onto the floor. But a tank drilled on the sides or back, up near the waterline, can only drain down as far as the bottom of the drilled hole; I drill as close to the waterline as possible to minimize drain-down in case of a power interruption. This makes it easier to ensure that the sump can contain the entire volume of the drain-down.
It's foolproof. As long as the cross-sectional area of the drains leading from those drilled holes can carry a water volume that exceeds the pump's maximum flow, there is no way to overflow the tank. And there is no need to "balance" anything. I currently have a pair of 2-inch drains on a tank that has been flowing a realistic 1500gph flow rate and the water level in the tank when running normally doesn't even reach halfway up the diameter of the bulkheads. I'm now experimenting with the addition of a DIY diatom filter on this system, which will run on its own smaller pump, somewhere around 800-1000gph. This increase of at least 50% in the flowrate barely moves the water level higher at all, as a couple of 2-inch drains can flow vastly more water than even the combined total of the two pumps.
Turning off one or the other pump requires no fiddling, and creates no changes in the safety of the system. Neither one pump alone, nor the two running together, can overflow the tank. Having 2 or more drains acts as a safety, a blockage in one drain (unlikely with a 2-inch diameter) simply means that the other(s) carry all the flow. And multiple drains reduce the volume of water that drains down in the event of a power outage, since the normal water level in the display tank is only slightly above the eventual drain down level.
No valves required in this set-up, although you can if desired place valves in the return lines from the pump(s) in the event that you might want to fine-tune the water flow rate. I don't bother with that, because I don't have any reason not to use full flow, but if you are a fan of complications the option is there.
Now, the fly in the ointment: noise. As far as I can tell, the sole reason for the existence of all the complicated overflow designs...Herbie, Bean-animal, who-knows-what...is that they run much more quietly? And, I suppose, if one is using an undrilled tank with HOB siphon-type overflows (but the very idea of such a thing makes me ill). These supposed "improvements" force you to figure out work-arounds just to get your tank to run properly, to balance the flow rates, to provide emergency solutions to problems that only exist because of the so-called improvements. Look at the suggestions made by
Backfromthedead
; they sound like they would work, but what a PITA just to correct the flaws in that original design!
Top-drilled overflows seem to run much more quietly than standpipes. My set-up has a vertical wet-dry filter right beside the tank into which the overflows drain. The main mechanical filter for the tank is right at the top of that tower, and can be swapped out in seconds. Bonus: if you get lazy and allow the mech medium to go uncleaned long enough to get clogged...the noise level rises dramatically as the water overflows the mech chamber and goes directly onto the wet-dry section, alerting you that it's time to clean!
So....here's what I would do if this tank were mine: turn off everything. Drain the tank down below the level of the weir and dry out the overflow chamber thoroughly. Remove the bottom bulkheads and silicone pieces of glass over the holes, or cap the bulkheads; either way, you're done with them. Drill holes in the back of the tank, either within the existing overflow chamber or elsewhere, right up at the waterline. Install bulkheads for at least a couple of overflows to the sump and whatever number of returns you wish, so I suppose two in this case. Attach all the plumbing leading to and coming back from the sump. When everything is dry and cured...rip that dang overflow chamber out and toss it. You get to keep all your pretty valves and straight white PVC piping with orange accents...but now your tank cannot overflow and requires no fiddling and diddling with flow rates, unless of course you just want to. In other words...it now works. You get more usable space in the tank as well, with fewer chunks of hardware in there taking up space and looking all techie-like. This is...or at least it should be...one of the benefits of sumps.
Admittedly, this is some work...and will be easier if you actually empty the tank and move the fish out temporarily...but it's a one-time PITA instead of the ongoing one you have now. It's very simple to set this up to pick up water from whatever level in the tank you wish, as well as with or without surface skimming if desired.
The tank can then be fun to own, which of course is the whole point. Is it fun the way it is currently? Good luck with whatever you decide!