Will this over flow system work properly?

No, it doesn't, but the water level in the second tank has to be below that in the first tank. The flow rate from the 1st to the 2nd tank will be proportional to the difference in water levels. With as small a difference as the diagram shows, the return pump will need to be throttled down to a very low flow rate to avoid overflowing the 1st tank. But, if that tank to tank water pipe is a large diameter pipe you would have a better chance.

I acknowledge that I may be wrong about this.

Hoppy said:

I acknowledge that I may be wrong about this.

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Yeah, the 2nd tank's water level is only a couple inches lower than the 1st tank, so yeah, the flow rate will be slow, and I'll adjust the pump to make sure it doesn't overflow the first tank. Thanks for suggesting the larger diameter there.

I wonder, do you think a triple run of 1/4" polyethylene tubing would be good? So maybe I'll use 1/2" in the first tank.

Anyhow, I'm concerned that the entire horizontal portion between the 1st and 2nd tanks will empty out when there is a power outage, and I can't imagine what's going to get the water over the tank's edge again once the power starts.

With the pipes turned to the water surface and no u-tube on the outside of a tank I don't think it will maintain any siphon at all.