I don't know that stacking speakers is a thing anymore. A lot of modern speakers are less than 8 ohms, so splitting outputs from an amp might cause problems (unless you are using two amps). Not to mention that imaging may be degraded by having having the directional drivers of both speakers separated in space from each other.

Back in the Double Advent days, most speaker designers were unaware of how great the importance of a system's horizontal and vertical polar response. Physicists were quite aware of interference cancelations, lobing, and other results from using multiple acoustic sources, but loudspeaker designers and those buying, selling, and reviewing speakers were seemingly unaware. If you look at speakers of the day, the tweeters and midranges were placed on the baffle in random locations with little regard to the interference caused by their relative placement. It wasn't until Joseph D'Appolito determined through experimentation the ideal driver spacing and crossover topology to "stack" speakers and not have unintended consequences.

Large PA system were also set up liked stack Advents. Back in those days, if one Altec, JBL, or EV system was good, a pile of identical systems was considered better... the results were mixed, but not usually particularly great. Today PA systems use "stacked speakers" in flown arrays that are purpose designed to augment each other and become single large line arrays with controlled vertical and horizontal polar patterns.

Thinking more about this, while what I posted earlier is all true, being technically better is not always the preferred choice.

In J. Gordon Holt’s 1968 review of the KLH Model Twelve he says, “And yet, paradoxically, for all of its naturalness of timbre, there is something missing from the Model Twelve. Perhaps we have just been spoiled by the extraordinary transparency of electrostatics, but the impression we get from the Model Twelve is that of a faint haze or curtain over the sound. There is just not the feeling of aliveness—of the lack of an intervening loudspeaker—that we have heard from some systems that are often more colored than is the Model Twelve.”

The KLH Model Twelve review was posted earlier this month and is an interesting read: https://www.stereophile.com/content/klh-model-twelve-loudspeaker

I have never heard the Model Twelve, but I think I understand what Mr. Holt is getting at. I believe this is similar to the experience many of us have when we listen to some of the vintage compression driver/horn Altecs and others like those that that Art Dudley enjoyed. Those high sensitivity systems like many electrostatics are extremely dynamic and while they may be more colored than many of today’s best speakers, many of us are able to get past that coloration and get the excitement of “realness” that an incredibly dynamic speaker can offer.

Every system is a collection of trade offs and compromises. By most accounts the stacked Advents were more than the sum of their parts. Those were pretty simple speakers that punched above their weight and still have a following even today. They are not everyone’s cup of tea, but that is why we have so many flavors to enjoy!

Mr. Widget wrote:

....Every system is a collection of trade offs and compromises. By most accounts the stacked Advents were more than the sum of their parts. Those were pretty simple speakers that punched above their weight and still have a following even today. They are not everyone’s cup of tea, but that is why we have so many flavors to enjoy!

I totally agree. It's hard to predict which technical faults or colorations are going to bother us and which aren't. It's not the same for everybody. I've recently played around with stacking the little Sony SS-CS5 bookshelf speakers because I had four of them. There are definitely lobing issues but if you're at the right height it works well and the sound is just bigger and more room filling than a single pair playing. I also tried them in a side by side arrangement, trying to make sure that they were distance matched on each side to the appropriate ear. This only works in one precise location but produced a very enjoyable sound that could be quickly altered in tone by moving my head forward and back. Kind of strange but really good in a way. If a recording seemed a bit too bright I could just find a head position that sounded best. LOL!