Going with one extension is going to be a whole lot better than going with two extensions pinned together. With the ball, you'd have three pins holding the mess together with all the slop these receiver hitches are notorious for. Heck, I bet the whole assembly would sag a couple inches due to slop in the connections. Then there's side-to-side movement.
So, here's what I'd do;
Go with the solid bar extension with the ball attachment that gets you the length you need. Also use receiver hitch anti-wobble stabilizers at the hitch and at the ball connection. This will make for a much more solid extension. The stabilizers look like this and you can find them all over Amazon with 2 day shipping. Might also be at Walmart.
https://www.amazon.com/MICTUNING-Heavy-Duty-Anti-Wobble-Stabilizer-Compatible/dp/B073PXQ6X5/ref=sr_1_8?ie=UTF8&qid=1541050062&sr=8-8&keywords=receiver+hitch+wobble
