I look at the dimensions and do wonder...the top of the Sub9 and the bottom of the 7 appear to be a potential match. Both are 20" deep and the bottom of the 7 is 14" across and the bottom of the sub is 17" across...but tapers as it rises.
Additionally, the Sub 7 net weight per speaker is 'only' 170lbs. The Sub-9 cabinet should be able to hold that...i have to imagine spread over 4 footers, that is 42.5 lbs per footer which is not bad.
Again, as you rightfully say, whether it was designed to be setup this way or not is another matter...but aesthetically it certainly looks doable.
You would know a lot more about Vandersteens...so i suspect you are right. But sure would be fun to enable space-constrained owners to be able to stack.
I believe the idea is also to distribute the low frequency, not only to add, as the dual powered (13 inch?) woofers dig deep enough on the 7.