Independent (un-manifolded) twin sets (doubles) scuba diving UK. Full face mask, redundancy.

This is not training, it is for information only. Get trained! This post shows a rig used for work and recreation based on twin indy 232bar 7s on a stab jacket with AP Valves' Buddy twinning bands. Streamlined, flexible, cost-effective, safe and pleasant to dive. Everything is redundant.
I can get two hour-long dives out of three 7s with gas in reserve, quickly and easily swapping in the third cylinder for dive 2, or two un-used cylinders if required. If I'm going deeper or working hard, I can twin a 7 and a 10 for 17l of gas. Or twin 10s. And/or side-mount a third cylinder for more gas, deco gas (on a half mask) or run air tools or an air lift. I can quickly break down my set into separate cylinders for transport - less mass to hump about, especially useful if (like me) you are a smaller person.
I show how you can use an indy set-up to run a full face mask by using a switch block without hoses trailing across your front or a failure-prone swivel in your FFM feed. You can use this set-up without the FFM on ordinary recreational dives to train your muscle memory and without freaking out a buddy - gas reserves and donation of a reg is exactly the same (from their point of view) as normal but you don't have to actually swap regs. This avoids the situation all-too frequently found on work dives: infrequent divers shoved into unfamiliar kit in an unfamiliar configuration, plus a FFM and who then have to try and do a job as well as run their dive. Shouldn't happen, but it does. What happens then in low viz (or ambient dark in the UK) when things go a bit south?
Note that the switch block is fixed in one easily accessible but unobtrusive place and in one constant orientation with a releasable cable tie and (on this jacket), the feed hoses are fixed to the rear with the velcro loops originally used for the (discarded by me) crack bottle for emergency inflation. The reg off the left post is not controlled by the switch block and this first stage runs drysuit buoyancy on this rig. Right hand first stage runs BCD buoyancy via first stage LP port. Connections to the switch block are (of course) at interstage pressure.
A pony cylinder as a redundant gas supply has big disadvantages. Manifolded twins also have disadvantages and are not the only 'twin' way. Take away the information, think it through with a pinch of salt, get trained and use the info' to get set up with a rig that best suits you and your diving (manifolded or not) rather than just follow a set path into twin set diving. Be safe, live long and happy diving.
June 2023: The OTS (made by Gull, in Japan) Mantis shown here is now in its 10th year of use on scientific diving jobs. Just done a job working several days at 25 to 28 m and another of several days at 31-32m, all on this rig on twin 232 bar 10s with no adverse issues at all. This mask genuinely requires no more air consumption than a half mask and is very comfortable. You can breathe through your nose. The two-taps switch block allows you to actually test the seal of your mask pre-dive at the surface, which I find very reassuring.