Sorry, I suppose to be the guy who triggered this duscussion with my posts about the way I used ny pony in a far past time..
I must clarify that I did never talk about "ditchibg an octopua" when using a pony.
I do not like the octopus.
Of course I had to use it several times, whenever tanks with double valves were not available.
But when tanks with double valves are available, I prefer to use two fully independent regs, particularly if it is a twin tank.
Most tanks here are single, steel, 15 liters, 232 bars, no reserve, convertible DIN/yoke. So this is what I can easily rent here, and what I use mostly (with two separate complete regs).
However, for demanding dives (solo, overhead, deco, strong currents, low visibility, etc.), or when there was some serious chance of being separated by my buddy, or when I was responsible of other divers as I was working as an instructor or DM, I did choose to add also a pony to my basic setup of a large tank with two fully independent regs.
At this point the choice is between moving one of the two regs from the main tank to the pony, or to simply add a third complete reg for the pony.
In the end I prefer the second option. This allows to give air to two buddies/students simultaneously (which happened to me in half a dozen of cases), or to continue the dive using the second main reg, when the first one fails (usually due problems at the first stage - and this is the reason for which I do not like an octopus, as in my experience the first stage fails much more frequently).
So I do not see the problem being discussed here: if the main tank has only one valve, so the basic setup is a single first stage and two second stages, when you add a pony there is no reason for keeping the crap octopus, it becomes substantially useless and can only become cause of problems (i.e., freeflowing and you cannot close its valve).
Better to leave only one reg on the main tank, and add another on the pony.
When instead the main tank, as in my case, has two valves and two fully independent regs, I would prefer NOT to remove one of them.
We are talking on adding a pony because an extreme dive is planned, so you need all the possible gas reserve and all the possible redundancy.
If the conditions are not so extreme, then I happily remove the pony, not my second fully independent reg on the main tank, which is also a decent redundancy for mild risk dives, without the inconvenience, weight and risk of entanglement caused by the pony tank.
So I use 3 setups only:
1) small tank (11l) with single valve and reg with octopus: diving at less than 10m, in super-benign conditions (warm water, no current, perfect visibility, proven buddy, possibly no suit and no BCD)
2) normal main tank (15l) with double valves, two fully independent regs: normal recreational dives with wife or sons as buddy (wearing the same equipment). This is what I did use in the last 30 years for most of my dives.
3) large main tank (possibly twin) with reserve and plus a pony, with a total of 3 fully independent regs: extreme dives with deco stops or overhead, or when responsible of a group of beginners. I did the last dive in this configuration more than 30 years ago.
I know that some divers use an intermediate setup, using a small 11-liters tank with single valve plus a pony, for dives corresponding to my profile n. 2).
I prefer my setup n. 2) for these dives, avoiding the bulky pony, but I understand that not everywhere you find 15-liters tanks with double valves, hence this hybrid setup of a small 11l tank plus pony (without octopus, of course) can be the only available option for getting some redundancy in such places.
