I'd go to the gym (and perhaps a personal trainer and/or a physical therapist) for some moderate conditioning knowing that it's very likely to improve my enjoyment of the activity.
By going to a twinset, you may relieve yourself of having to switch tanks but you're going to incur some new challenges with valve drills given your dexterity.
I'd keep it simple and work on getting yourself as fit as possible for the activity rather than pursuing a more complicated equipment solution.
Also, get a beach cart with fat wheels for the sand slog.
Definitely looking into the beach cart. Next time I'm on the ferry I'll see how many steps, and how narrow things are, but I'm actually pretty glad someone suggested that because I did not think about it!
I'll address the physical bit below.
You may want to slow down and put another 100 dives on your gear and if you buy a second tank just like the one you have the switch on the boat won’t involve as many changes.
Doubles involve much more than a manifold, you’ll need tank bands and most likely a backplate and wing to clamp it to, I know some B/C’s claim they can take doubles but it’s usually not a good match.
Where are you diving, what part of the world?
I'm diving in North Queensland, Australia, but in a small town away from any tourist spots. Which is why aluminium cylinders are so pricey - nowhere that sells them offers a good freight deal. All my dives are shallow, 10m or so and at the moment there's not much to see due to huge algal blooms. There's one instructor, but he only offers OWD & AOWD. Most of my dives have been overseas, but I like to keep current so am diving once a week. I'm feeling myself stagnate at the moment as I don't have a goal to work towards.
Now I think people are reading a bit too much into my original post - the bit about strength was meant to be levity/self deprecation. I'm a perfectly fit person, I just look a little stronger than I actually am. I have slight tremors in my hands which gives me dexterity issues - but again they're a minor issue for everything except tying knots which is hugely troublesome for me. I don't expect any issues with shutdown drills (though I've been wrong before).
The reason I am deciding to go twins is mainly "why not?". I want to buy cool stuff to further my hobby, and I was deciding whether to go for a DPV or a twinset as both cost about the same, and twinset won out because it fixes some minor inconveniences in my life whereas a DPV is just cool. I already have the backplate & wing, just need the tanks, bands and manifold.
I appreciate all the responses, I really do but I'm just really looking for advice on what particular cylinder I should go for twins on!