Its gonna be hard to reach equilibrium. Either tank is going to end up with more water over time. ..And end up on the floor.
I would just Get a reef ready tank with 2 bulkheads in the weir. Or just drill/have your tank drilled. Some guys are brave and can pull it off with the tank half full. With primary and emergency drain(s) your as close to fool proof as you can get.
Its easy to drill a tank. If I can do it, you can do it. One time I drilled a 5.5g with a 2.5g sump to put on my desk at work. no special tools, just a crappy hand drill, a garden hose, and a Chinese hole saw from amazon. And that's on ultra thin glass. On the heavier stuff 40g and up its alot easier and more forgiving
Overflow boxes are good too, but have disadvantages that a drilled bulkhead dont have.