I dive twin 7's that's doubles with 7 litre cylinders. i find this perfect for solo diving, i don't usually dive more than 30m as i get bored doing deco. i can't quite understand the concept of needing to look at my cylinders during a dive. in nearly 20 years i've never had to do so in my basic training i was expected to remove my whole kit underwater & replace it so if i wanted to admire my cylinders i might be a bit rusty, but i could still do it.
i always carry a slate, i make notes on every dive and the info goes in my log. Not wetnotes i hasten to add, but a home made slate.
Looking at the cylinders is not for admiration, it is actually not so important. What I think is important for a SOLO diver is to be able to see and control his 1st stages on both tanks, and the ability to ditch/dismount&mount his gear in order to easily solve problems. Sure, you can do anything with a set of doubles w/manifold, but at what cost- in terms of easiness, and time to solve a problem?
Here is an incident that happened to me while solo diving with Side-mounted tanks:
Started the dive normally (checking gear, etc) to a dive site in which the zone of 25-30m has lots of speed boats activity (jet skis and the such) and glass bottomed boats too, so basically going up in emergency is not recommended, and better be avoided unless it is a "when the sh!t hits the fan" situation
To make long story short, after about 30minutes my right regulator started free flowing. Now, a free flowing reg typically empties your whole tank quite fast- modern regs with high flow rate it can be a matter of less than a minute, not to mention that the tank was not full by the time the free flow started.
Most times it is 2nd stage problem and can be solved quite fast. However, this time it didn't help, so had to close valve.
So, if the tanks are on your back, you have to start mingling with valve drills, isolating between tanks and also locating and closing the valve for the free-flowing regulator, while all the time it keeps bubbling and free flowing like crazy. How much gas do you lose in the process, if you manage to do it at all without buddy assistance? For me, it was always difficult and I never felt easy with this action, and with some dive suits it and/or gloves, it was almost impossible (for me, perhaps other people are more flexible and talented and find it as easy as walking).
But, I was on sidemount, so I just had reach my hand to the first stage valve and turn it off- approximately *5 seconds* and amount of lost gas- *negligible*. As I was also managing the gas between the tanks even if all gas was lost on right side I could still have all what I needed to end dive safely on the left side...
But no gas was lost, and after these 5 seconds that valve was shut I had all the time in the world to assess situation, find problem, solve it and decide whether to abort dive safely etc.
FOR ME, it was the difference between doing an extremely simple action and no losing of gas, no effort whatsoever, no need to remove any gear- nothing. As simple as that.
And, suppose I also had the need to remove the gear, say, to undo entanglement with fishing lines (happened to me on the very same dive mentioned above), then I just had to unclip the cylinder which got hooked, cut the darn fishing line, and go on with the dive. With the doubles on your back, and the fisherman pulling like crazy thinking he got some big game fish- could you actually do it? Damned Murphy, the fishing hook always gets you in places which you can't see or are hard to get too.
Everyone can dive with doubles, be a wizard with manifold handling and all the drills etc, but I think for a solo diver it is more important to be self sufficient, flexible to move easily, able to remove whatever piece of gear, and also able to easily control first stages with a minimum of time and effort.
Because, if the need to solve a simple failure puts the diver to some stress and/or effort, any second failure may start the way to a perilous situation.