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So first off, here's my lane balancer. As far as I know this allows 100% throughput (just a bit big).
And here's what I'm calling a 2-Belt Overflow Balancer. It will keep the inputs on their output belt (top stays on top, bottom stays on bottom), and will only actually balance between the two if either are backed up.
And lastly, here's two lane balancers, followed by the overflow balancer:
