Let's talk about plumbing...
I know almost nothing about plumbing - I'm pretty sure it involves water, swearing and an Italian that likes mushrooms and money. When I raised my thoughts about this setup (two tanks, shared drains, a horizontal run of about 12 feet) in the DIY forum they politely suggested that I refrain from doing this. Obviously, being as dumb as a box of rocks and twice as stubborn, I decided to do it anyway because clearly my own intuition is superior to the combined knowledge of the entire connected world.
So - what do we have...
Each tank has two returns - one main and one backup. The main returns both drain into one 1.5" line, and the backup returns (which really dont do very much except in emergencies) both drain into a second 1.5" line - it is basically a shared-double-herbie system. The main return from each tank starts out as 1" flex PVC, with a gate valve between it and the 1.5" flex PVC drain. The backups are the same, except they are more direct and don't have the gate valves, since I want them to make a lot of noise if for some reason they ever start to be used.
Left tank drains:
Right tank drains:
Both of these drains head out to the sump on a *slightly* sloping 12' of flex PVC run through the wall and into the sump in the unfinished basement.
The returns head into the tanks via two lines (each with their own return pump for redundancy) that rather conveniently fit in the space between the two tanks, and each return splits and loc-lines into both displays. Basically, a failure of either return pump drops the water flow to both tanks, but I will still have water running through both displays:
The plumbing through the wall is messy as hell, so I constructed an extremely expensive (about $20 in materials - 1/2" hardwood plywood, 1x2's and yet more plastic trim, with more neodymium magnets holding it to the wall) plumbing-hiding-thing that doubles as a seat for the cats as they try and figure out what the hell I am doing.
This is what it looks like raw:
And this is what it looks like with my patented "plumbing-hiding-thing":
I fully intend to nail some more trim to it.
I am equal parts happy and amazed to report that this system appears to be working exactly as intended. With some minimal tweaking of the gate valves, I now have almost entirely silent plumbing (the MP40 I have in the tank is significantly louder than the drains). I have tested every failure scenario I can think of (failure of main drain in left side, right side, both sides, partial drainage through both etc) the backup drains pick up the slack and noisily alert me to the fact that there is an issue. I'm not running enormous flow to the sump and back though (maybe 1200 gph through both tanks), so don't blame me if you try it yourself and it doesn't work - this isn't a how-to thread, more of a train-wreck-in-slow-motion thread with a few pictures and some drunken commentary thrown in.
Dave