The outside container requires a dividing wall, where one side drains to the sump and the other maintains a constant water level to hold the siphon.
The specimen containers aren't very durable and crack easy when they're drilled (for the drain plumbing to the sump) and/or fashioned to support heavy weights (plumbing to sump and water).
The inside-the-tank box needs a weir notched into it to provide some "sifting" to prevent large material from clogging the U tube.
A 400gph powerhead may only have 80 to 100 gals of flow at the header (distance from sump to the top of the tank). Simply adding another pump will not greatly increase the flow (header and return pipe friction at increased flow). {Tie 2 birds together and, 'though they have 4 wings, they cannot fly}
Good idea but, why try to reinvent the wheel? You can find used overflows for as low as $5 on ebay.