The sponge filter thing is a nonstarter, set them perfectly now, go watch some TV, and come back and see if they're even remotely the same volume in half an hour. If you want to cascade them together, you can either plumb a straight bulkhead from one to the other through the side (with some screen prefilters on it so nothing can move from one side to the other) or cascade from one higher tank into one lower tank. Think about a circuit a->b->canister filter->a instead of a->b == b->a because you're not gonna get that without some fairly sophisticated flow regulation/level sanity checks.
or you can do it in the back.
another option: trim those middle sides down enough to glue the acrylic bridge between 2 tanks. this allows water flows back and forth.
anyway looking great.