Great work so far. I would second your idea to put a removable center brace. I am paranoid about sag over time but you would still have the access when needed.
Early on, I had two tanks hooked together with bulkheads to essentially make one sump. If you size the bulkheads right, you might be able to get away with it, you'll just need to do some testing and make sure your flow through doesn't exceed the rate the plumbing can handle. I would plan on multiple large bulkheads. Also, I might be stating the obvious, but you'll need two bulkheads for each connection. You can't just clamp the glass together between the two tanks with one bulkhead. Does space allow for the two bulkheads and short connecting pipe? You'll need to have the holes lined up fairly well unless you can tweek the pipes joining them. I don't drill glass. I know it can be done, but it seems like it would be tricky to get those holes lined up well enough.
Bottom line in my opinion is, yes it could potentially be done, but a lot of things have to line up. You might be better off looking for other solutions like waiting for a used acrylic tank to pop up here or in Craigslist and modify it to your purpose. Cut it down, add baffles, do whatever you need to do. I find acrylic easier to work with in that regard. It looks like you already have the carpentry tools needed to do the job.
That's just my opinion. Good luck and keep up the good work.