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I did something similar but didn't have the headroom in the basement to space the two tanks apart enough to get in and weld. So I cut 3 holes (about 5" X 5") along the length between the tanks and butted them together. I joined them with a skirt of 6" wide steel but hydraulically it's the same as what you're talking about. I just used a single port for both the supply and return to the boiler but they are 3" pipe so the water will be moving slowly as it goes into the tank to minimize mixing.
While I was tinkering the thing together I found myself fixating on things that were probably trivial in the long run. Heck, there are folks out there using propane tanks horizontally with the vertical-entry stock fittings and they 'work'. They stratify. Others are using two tanks on separate pallet racks joined with flexible hose. They work, too.
Getting the ports closer to actual top and bottom and using larger pipe to make the water slow down before it enters the tank (or using multiple fittings as you are proposing) helps to minimize mixing. And remember that once the supply water from the boiler pushes enough hot water into the tank that whatever turbulence you have is surrounded by water of the same hot temp, mixing is a moot point. It doesn't continue to mix throughout the entire boiler firing cycle.
I don't thing anything about this homemade heat storage stuff is conventional. Some of us have to try these things.
But let's all report back after a few years for the interest of others.
