A hang on overflow would be a million times more reliable than some contraption with having to match flow. There's absolutely no way you can reliably use two pumps to do what you're looking to do. You'd have a full siphon going from the fuge to the tank, and the pump from the dt would have head height to deal with... not to mention the possibility of something stopping one of the lines either way and dumping no telling how much water out.
The ONLY way you can reliably make this work is to drill the refugium and have it overflow in to the DT.
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