Is it possible to connect the two sump containers with a piece of pipe? Here is the problem...you will never be able to balance the flow perfectly. Even if you get it working just right, your pump flow will change a little as water levels in the sumps change or as a bio film builds up in the lines. Using ball valves sounds like a good idea until a snail, who would otherwise pass on through to the sump, gets hung at the valve and restricts the flow. If you could connect the two sumps with a couple of bulkhead fittings and some 1" pipe, the water level would always equalize in the two and you wouldn't need to try to balance flow anywhere. Anytime you try to balance a pumped system using valves, you're asking for water in the floor. JMHO.
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