I believe the building-block approach you're using has some merit. A pair of larger 1x12 cabs and a separate head would probably perform better than two of what you already have, but if you like the sound of your combo and simply want more of the same, then by all means proceed. Yes, a pair will be noticeably louder. Just not way louder.
Standing back a bit, this may be an opportunity for you to step up to a new class of hardware, rather than simply duplicating what you already have. I'm not recommending you do that, just that you seriously consider that option in light of where you plan on being two or three years down the road. It's easy to shed a 1x12 combo because it's not loud enough. It'll be tough to shed two or more 1x12 combos because they turned out not to be loud enough. It's the concept of incrementalism. Incrementalism can put you in the poorhouse because your upgrade steps are too small and too frequent. I believe you should always buy what you plan on needing tomorrow, not what you need today.
Years ago used two identical combos (SWR Baby Blue II's) that worked ok for me but never seemed very efficient. These days with such high powered amps being so small it seems more cost effective to get something like a 600W @ 4ohms Genz Benz Shuttle 6.0 1x12 combo and just add a separate unpowered (lighter and cheaper) 1x12 cab.
Another option would be to get a cab that matches yours and slave a power amp like an SWR Amplite to your existing heads preamp.
Try two amps of some sort before you shell out your cash. It probably doesn't work as well as you're hoping it will.
Since the shown combo can drive an extension speaker there's no point in paying the cost of a second amp. Ideally the manufacturer would sell an extension loaded with the same driver.
I wasn't familiar with this amp. It appears to be an entry-level model, made in China exclusively for Thomann. Checking Thomann's website, there's no sign of an extension speaker OR a manual for this amp. So, no matching extension speaker, and no way to know if the extension speaker jack is a parallel or series connection.
Also, isn't the OP's combo 110 watts? I need more than that all by myself(DEAF as a STONE ). Kidding on the deaf part, but I would suggest setting your sights on more overall power than the 220 watts 2x of these combos would give you.
twin 12" combos? 
). Kidding on the deaf part, but I would suggest setting your sights on more overall power than the 220 watts 2x of these combos would give you.
