[mike31154]

What abcha0s said, I'd also recommend beefing up your return pump & plumbing the drain/return for each tank separately. It's a bit difficult to wrap my head around how exactly you're trying to accomplish a siphon between the two tanks and I have no idea what could possibly go wrong, but it does sound a bit complex & fiddly.

I've purchased two external return pumps in anticipation of a future basement sump build so they're sitting idle right at the moment. If you'd like to try one or both, lmk & you can see if either type would be something that could do the trick. That'll give you a starting point for what you might need to purchase with regard to a pump, although they all have head pressure graphs and since you know you have 15' to overcome, you should have an idea of what's out there.

I have a Panworld NH-150PS (good for over 20' head pressure) and a Poseidon PS4 which is less powerful than the Panworld but is still able to handle into the mid teens of head pressure fairly well. My plan is to use the Poseidon in the winter since they are known to add a fair bit of heat to the water despite being an external pump. The Panworld would be for use in the summer when there's less need for supplemental heating and it can become a challenge to keep the water temperature down.

Edit: The new tank can't go down where the sump is? Would make a nice refugium or second display down there and your plumbing issues would be solved fairly easily. Plus you wouldn't be blocking one whole side of your nice 3' tank with the 6'er. Not to mention the extra weight on your floor....