Hmmmm. Not sure how many manufacturers, if any, provide two cylinders with their guns - I've never heard of anybody doing it, but then, I've only been BP shooting since the late 1960s. Others with more experience of the rather odd laws hereabouts might care to put me right with regard to additional cylinders, but in general they each require an entry on your FAC as a independent and stand-alone item of gunnery.
I'm not up to the competitions that you shoot here, either, but the MLAGB does not appear to have any competitions that require a rapid reload using another pre-charged cyinder [minus caps].
Given the odd take on any kind of shooting that is outside the norm, I'd check with your FEO about the feasibiity of acquiring another cylinder, and what 'good reason' you might have for justifying it. Certainly, it is a pressure-bearing component of any revolving BP firearm, and so you can't actually just go into a gun-store and buy one like you can in the USA. I'd say that each case is taken on its merits with regard to the number you might have in your possession, and that each one is treated like an extra barrel for your rifle - like the SAKO quad, ror instance.
tac