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Moving to the Tank Talk forum sounds good! (I didn't know it existed, sorry.)
The project is coming along pretty well. I've updated my design and my 210g and 75g have arrived (attached, but I've recently decided to just use one return pump and two smaller heaters in case one fails). I'll be contacting a local glass company this weekend about getting some baffles for my sump and then I should be ready to start adding RO/DI water and live rock. I'm still on the lookout for a good non-metal mesh-like material to cover the top of my overflow.
I would recommend not joining the overflow pipes into a single, blockable tube. Unless the tube receiving the two outflows is larger than the delivery tubing you will be choking your return. Additionally, combining the tubing allows for a single point of failure if blocked by debris or an energetic (voice of experience) snail.
If you have decided on a single return pump, check into using a SCWD (or something equivalent) to still give you return flow from two different directions.