1.Is there such a thing as recreational diving on twin tanks?
Absolutely. Whether you use one set of doubles for two dives (as suggested) or do a somewhat extended single dive that keeps you within NDL, doubles are NOT exclusively associated with 'technical' diving.
2.OK, I accept that there exists "doubles recreational diving" but.. how popular is it? I don't want to pretend to be "tech", or, as some people might think - show off - by using twins.
In part, the answer is influenced by where you are. Off the NC coast, where you are looking at a 2 hour boatride to a two dive charter on the enjoyable wrecks, I frequently see divers (and am one, myself) who take one set of doubles to complete two recreational dives. You would not be looked at as someone who is showing off, or pretending to be 'tech'. I cannot speak for other areas of the world.
theriel:
Can I easily rent doubles everywhere in the world? Basically - what types of questions should I ask myself before buying either single or double tanks?
Generally, you cannot easily rent manifolded doubles. However, you CAN rent two single tanks. And, although (double tank) sidemount diving has been around for 5-6 decades, it is rapidly gaining broader popularity now, as a lot of equipment to support sidemount diving has begun appearing on the market in the past several years, and sidemount has moved out of the canister light of caves, into the sun light of day. I think that may turn out to be the easiest path into doubles diving in the near future. You can buy individual tanks and use them as singles, then use them together for sidemounted doubles. So, while a BP/W is a good suggestion for single tank and backmounted doubles diving, and one I used to make routinely myself, you might also think about a rig that would support both single tank diving and double tank sidemount diving. Just yesterday afternoon, I was in the pool with a dive buddy who is a very experienced diver, and fellow instructor, as he was familiarizing himself with his new Nomad sidemount rig. He commented on how much easier to use, and more stable, his sidemount rig was compared to his back-mounted doubles.
theriel:
2.Could somebody explain me what BP/W *really* consists of (I mean, which parts do you usually buy separately)? Is it the wing + harness + plate, or is it just 2 part -> plate (with harness) + wing?
You can purchase all three together, or purchase a BP/harness together and a wing separately, or purchase all three separately. The latter approach is my preference / recommendation, but there are many good 'deals' on a single package.
theriel:
3. ... is it possible to buy a system both for doubles and single tank diving?
The BP and harness should serve both, quite well. You are best served by having different wings for single tank (smaller) and double tank diving.
theriel:
4.What is the difference between Aluminium and Stainless Steel backplates? (yeah, I know, the material, but... practically - why would you choose one over the other?)
The selection depends on your exposure suit, your intrinsic buoyancy (which may reflect what exposure suit you are using, among other things), and whether you are diving fresh or salt water. It may also reflect whether you are traveling with your rig to dive destinations. The SS BP is heavier, some find it useful for single tank diving, and double AL tank (or even double HP steel tank diving), DRY. Since it is heavier, it may create a baggage weight limit violation in air travel. Others find the AL plate well suited for wetsuit double steel diving.