Please distinguish between first stages and second stages. Just saying "reg" is unclear.
One "reg" is a first stage plus a second stage. I call an additional second stage only an "octo", not a reg.
All my regs are the same (SP MK5+109). The "main" reg differs form the secondary because it also carries an hose of the BCD and the SPG, while the secondary has just one longer yellow hose, connecting the first and the second stage.
Here in Europe the standard tanks for rec diving are 15 liters, 232 bars, with two posts, so you can always use two fully independent regs on two posts.
I did find similar tanks in all diving centers around the world which were managed by Italians (Maldives, Kenya, Tanzania, Seychelles, etc.).
Only once I did not have the possibility to use my two independent regs, and I had to dismount one secondary and use it as an octopus for the main reg.
When I was working as an instructor and dive master at Maldives, the customers were given smaller 10-liters tanks (never understood why) so they were very often low on air, and we had to give them our secondary reg.
In such situation, I was using two first stages and THREE second stages, the third one being mounted on the left shoulder, so "wrong" for me but easier to breath for customer. And it happened to me to use both secondaries simultaneously, giving air to customers which were low on air, while ascending slowly to the rod where the deco bottles were attached.
Last point. I am NOT a tech diver. However, I am qualified both as a diver and instructor to operate down to 50m, with deco (and to use CC pure-oxygen rebreathers). This was (and is still) rec diving if practised in free water (not inside a cave). The whole concept that "deco si evil "or "deco is tech" is completely wrong, in my opinion: a dive planned and conducted just beyond the NDL limits, where a few minutes of mandatory deco stop are planned and required, is safer than a dive planned to be just within NDL; where any minor unplanned event can push you outside NDL, without being prepared and equipped and trained for deco.
Back on topic about pony/deco bottles. In that period at Maldives we were using 10-liters deco bottles, but we were not carrying them with us for the whole dive. They were provided on demand by the support dhoni boat, when the DM launched his buoy (at the time it was a round buoy), splashing in the water a system made of two buoys, carrying a steel rod at 3m, and two deco tanks hanging down to 6, (where the first deco stop was occurring in the worst cases - in most cases we were going slightly in deco, so only a stop at 3m was required).
Only in very rare occasions, when there was a high risk that the boat could not provide the deco bottles on request, we had to carry them during the dive. In those years proper harness for attaching them to your body in a streamlined fashion had not been invented yet, so typically the leading instructor and the DM closing the group were carrying one of these deco bottles simply attaching their valve to a shoulder ring with a carabiner, and leaving the tank and the reg hanging around. Not nice, and very unpractical...
However, while carrying such an additional tank, it had been barely stupid to remove the second reg form the second post on the main tank. I really do not see the point raised by @
tbone1004 that when carrying an additional tank (call it pony, or deco, or simply additional) you should remove the second reg from the main tank... The main tank, with just a single reg, is quite unsafe. If this reg has problems, all the air trapped in the huge main tank becomes unavailable...
And doing a CESA had never been a good solution, for me. I did never teach this to my students, I always recommended to solve any problem staying underwater, and to carry equipment and being trained to use it for avoiding the need of a CESA.
