Hey man how ya been ? The second drain is a dry Edrain.What type of drain are you planning to implement? If you have one main drain, I'd consider sending the second through a filter sock first for mechanical too.
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Here pictures might help.
NVM the marine lighting that is going to be replaced
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One last thing you can drill a siphon break just above the water line on the return in the main tank to stop back flow when the system is turned off or you lose power. Also can add a check valve near the pump side of the plumbing
I would also suggest that you put your media in lingerie bags or some such for easier maintenance

I'm not sure I understand your question.
The second one is purely a back up overflow in case something goes very badly and the first overflow becomes blocked.
The picture helped. So the overflow drain set-up looks to be a "Herbie"? You have a full siphon and a second emergency drain. I have a similar set-up but I typically have a trickle in the second drain and completely dry third drain.
Anyway, what I suggested was to have the edrain go to a sock rather than directly to your biological section. But I see how you may want that to raise attention if your e-drain is taking in flow?