Look at it from my point of view, one person says one thing, I try and understand it and then another person says it's incorrect anyway!
The problem is that there are many ways to achieve the same thing. There isn't one way which is "correct", just different sets of tradeoffs. Even the best heating engineer will have personal preferences which will bias the solution he recommends, and of course there are a fair few in the trade who really only understand the "stuff they use personally" (you've already found this trying to get someone who understands a thermal store which is in fact much simpler than a combi boiler
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1) If you have two cylinders I take it that it's not difficult to get them to run sequentially? In other words the first cylinder gives it's 210 litres for the first two 20 min showers, then when the temp drops water is taken from the second cylinder? I take it this is the whole point of two cylinders?
I think everyone is suggesting they'll just be connected in parallel to get a larger capacity. From comments, I'm getting the feeling that there are two reasons - two smaller cylinders being "common" are cheaper than one larger cylinder; and the coil capacity vs volume ratio is higher for the smaller cylinder so 2off smaller cylinders can reheat faster.
2) That then just leaves me to figure out what would be better in terms of recovery time, two 210 cylinders or one twin coil 500 litre.
As above, from comments it sounds like the bigger cylinders don't have heating coils which are scaled in capacity as the cylinder size increases. So while the bigger cylinder will hold more hot water, it will reheat more slowly because it's coil isn't as powerful as 2x the coils in the smaller cylinders.
On the "twin coil" question. Some people are suggesting they'd use the twin coil cylinders and connect the coils in series. This means that if (say) a cylinder had a 30kW "boiler" coil and a 20kW "solar" coil, putting them in series would (in theory) give you a coil capable of transferring 50kW. I think someone suggested the smaller cylinder came with twin coils as standard but the larger ones don't - so that would be another reason for using 2off smaller cylinders.
2 smaller cylinders will have a different space requirement as well. While less efficient in space utilisation, 2 smaller cylinders would take more length along a wall but would project less - so rather than having to size a room wide enough to work round the big cylinder and have excess space elsewhere, it may allow you to have a narrower room (there's also the issue of fitting larger cylinders through doorways/gaps if you need to replace it later).
So many variables, no "right" or "wrong" answers, that's why you get so many different opinions. Your requirements could be made to work with many options (subject to things like gas supply limits) - combi, vented cylinder, unvented cylinder, thermal store, heat bank, single vs multiple cylinder, single vs multiple boiler, etc, etc. None are "wrong", just different tradeoffs - and everyone has their own ideas as to why their favourite is "best".
