<font color='#0000FF'>My two cents worth
Twinsets: obvious advantage is more gas but more importantly redundant air supply. with independants you can get going more cheaply by using soft cam bands with your current BCD. How much lift in your BCD ? Stuff like the Buddy Commando can handle twin 12s, (though like most here I prefer a wing) so going that route could get you twinned up for the cost of an extra tank and soft cam bands. But in a FUBAR you don't have access to the air in an independant if you'ge got a terminal free-flow.
Manifolded twins with no isolator - can shut down a free-flowing reg but could still lose gas if there is a problem with the pillar valve
Manifoded twins with isolator - the safest option and IME the commonest confiuration; even in the event of a PV problem can still isolate and save the gas in t'other tank.
Courses - IMVHO, absolutely not essential in the
first instance; I got my twins and wing and started using them before doing stuff like Deco Proc course but still diving in the <30m range. It's actually easier (IMO) than diving 15L + Pony as twins sit better on your back.
The "experienced" guys in my old club used to treat 'diving twins' with the same mystique as doing an untethered spacewalk - as long as you're comfortable with the extra weight of the kit it's no different to diving singles. Having said that I once saw a "new-ish" Dive Leader using twins (12s) for the first time and he couldn't even cope with his kit well enough to look after his trainee, which was bloody irresponsible.
For going deeper I'd say most divers would benefit from extra formal training, I (and others here) can recommend the TDI Deco Proc course. Are you nitrox trained? if not might be beneficial to do a combined nitrox/deco proc course.
Some divers (I believe Mark Chase is one) limit theirselves to 45M for air and prefer to use normoxic trimix for +45m. Personally, I see myself going that way for +45m dives in this country, even though my BSAC limit is 50m on air, I just don't want to be narked at that depth.
Just for comparison my setup is:
2x10L 232bar with MDE isolator manifold (got them S/H , a bargain at £220 just tested for first time
)
OMS 100lb "Batwing"
single bladder, red and bungied
OMS "IQ" harness (the newer pocketless version)
Twin Tx40s one on DST 1st stage, one on DS4, long hose on primary, normal length hose on secondary,
Single SPG
normal length BCD inflator
ND Schrader "Blowgun" drysuit Valve and Hose (for filling dSMB etc)
3X Sherwood tactile knobs
apparently unbranded SS twinning bands (I can't see any logos)
CD Valve guards
All of which is inverted 'cos I don't have Gibbon-length arms like Gav and like the ease and speed of being able to shut-down in an instant
HTH
Steve
Added later in relation to Angus's post - I don't what diving you've done to date Duiker, but I have (rightly or wrongly) assumed that you've done at least some wreck dives before this point and know how to deploy a dSMB.
I personally don't think there's any mystery to finning around an artificial reef (which is what most wrecks are to me), and I've certainly done "first OW dives" with trainees where it's been a shallow wreck, IIRC I did The Breda (about 15-28m) with about 10 logged dives.
Of course wreck penetration would be a whole other ball game, but even then, it's not rocket science to lay out a line.
It's gets a bit more challenging when you're diving wrecks from a RIB (or even a smaller inflatable) and have to locate the wreck with transits and echo sounder, shot the wreck (an art in itself) recover the divers etc etc. If you're a BSAC diver most of this (plus other good stuff) is covered in
Advanced Diving Techniques which I can highly recommend