There appears to be an explanation at
Le tuba à double tube de l'association BONITO e.V. Par le Dr L. Seveke in French, which I have translated below:
The double-tube snorkel from BONITO e.V.
By Dr. L. Seveke
In the 1960s, a somewhat odd-looking swimming device haunted the "diving circles" of the Potsdam district of the GDR - the much-derided and admired BONITO snorkel.
We laughed about it, because as a "real" finswimmer, you naturally didn't need to use such a "deer antler" to perform, and we admired you, because you didn't know exactly which trick was behind it and also because it came from the mysterious BONITO eV.
It was a club run by biologist Wolfgang M. Richter, who wasn't in the GST (Society for Sport and Technology - East German paramilitary sports organisation) (very strange!) and did his own thing in the Baltic Sea and later in the Mecklenburg lakes diving and independent scientific work on water bodies.
But one thing they had in common with us was that they had even less material than we did, at least some of which we could use thanks to the GST.
So they also built their own, creatively and successfully. The double snorkel was one of them, but also cameras and compressed air and rebreather diving equipment. They even managed to have certain devices produced in small series under GDR conditions, the Tauchtax camera from Curow in Dresden (merged into VEB Pentacon in 1970) and the snorkel from VEB Degufa Berlin.
However, W. M. Richter told me the other day that the production of the snorkel was not done by them at all.
This double snorkel was therefore put into circulation in a few copies, but it is now a very rare collector's item.
The design was primarily intended to prevent pendular breathing, which could already cause headaches during extended shifts due to the thin PVC tubing that was only locally available. Using two flap valves in the mouthpiece, one tube was used for inhalation and the other for exhalation.
Also for reasons of stability and resistance to wear, the two tubes were joined together above the head, where the air exited through a sealed valve for tennis balls.
BONITO apparently used the double snorkel for years because it suited their scientific work perfectly.
It was not as successful with the general public. Then, a few years later, there were enough thick PVC barrels for snorkels, so pendular breathing was less of a problem.
It was too bulky for the finswimming competitions, so we preferred the sleek centre snorkel.
For me, the double snorkel remains an interesting symbol of homemade equipment that is meticulously built for people's needs, by innovating.
As an example for the work of BONITO e.V.: Biologische Station Feldberg t1p.de/lpyt
When I walked on the BOOT a few years ago, I suddenly believed in déjà vu. The double snorkel was back!
Of course, it was hailed as a totally new invention, with exciting and never-before-seen benefits ;-).
Admittedly, it didn't have so much in common with the good old BONITO. The design and manufacturing technology differs anyway, but so does the purpose of its use.
Pendulum breathing is prevented by the exhalation valve located directly in front of the mouth, as is already the case with current single-tube snorkels.
And today's divers are used to bubbles bubbling in front of their faces from their one-tube snorkels.
But the section of the inhalation tube is practically doubled, which reduces breathing resistance. And since we all do high-level sports these days, this is absolutely necessary, like the carbon bike for the pensioner.
The symmetrical design, however, improves wearing characteristics; there should be no flapping as with attaching the single-tube snorkel to the mask strap.
Even high-tech diving masks are now equipped with double snorkels. It sure makes you the best at the pool.
Let's see what happens next.
DRW