In all of my "horse years", let's say, 18 of them (with my 15 year break in there of course!), not have I only never seen anything such as this until tonight, but also, have never heard of anything like this!
I suppose in order to use such a saddle, one would require a horse with a long, yet simultaneously very strong back--not too common, right? Perhaps a full-sized draft?
And have the saddle be fitted just so, such that, of two "seats", each would be just large enough to hold one very small child, with a second positioned directly up against and behind the first as pictured.
If this was the case, and the total weight and approximate bodily distribution of mass of two very small children, riding positioned via the saddle in a very close together manner, and with the saddle designed to distribute their total weight in about the same place a single, larger-sized adult would have his or her weight distributed, I SUPPOSE it could work.
IT just seems VERY odd to me...not being judgemental, I simply...well, really find it to be ODD ! It doesn't seem to have a very wide application or range of usefulness, other than to allow two small children the added security of having a saddle to "hang onto" while riding at a very slow gait (walk??) on a safe mount. Thus, perhaps. Showing the children that "together you needn't be afraid to ride"??
In my opinion, there are likely much more efficient and practical ways to get kids to feel safe on a horse the first few times than to put them on horseback together in a "Double saddle" and make some poor horse cart two nervous, oddly distributed (weight/placement wise) squirming kiddo's around!