I expect there would have been a lot of factors responsible for BSA's reluctance to produce twin-carb heads. For a start they didn't have any positive experience with road-racing which is so often a catalyst for performance-enhancing parts/models (triumph brought out the bonnie and daytona tiger 100 off the back of racing successes). Furthermore, the company were reluctant to encourage road-racing in case any losses would dirty the BSA name - so reluctant were they that they may be seen to have actively discouraged the practice by producing less racey parts.
In addition, it must be remembered that a lot of what the British motorcycle industry relied on was fashion, and it was often Triumph rather than BSA who set the standards. When triumph released the bonneville in 1959 it set a trend and demand for twin-carbs. Norton and Royal Enfield dually followed suit, but it took bsa several years to release a twin carb model - a fact perhaps explainable by their desire to introduce and ground the new 'unit' models. i.e. why spend money making dies to manufacture sporty bits for an antiquated (pre-unit) model, particularly at a time when so much expense was going into tooling up for a50/a65 production? Some might argue they should've released a twin-carb unit model straight away, but BSA seem to have always been conservative, at best, and probably wanted to ensure their name for reliability remained intact at least for the first few years of the new machines.
Which leads to another explanation - conservative management. The old adage 'if it ain't broke, don't fix it' fits perfectly with anglo saxons in general, but i reckon the british motorcycle management took it to extremes.
anyway, that was quite the essay, i hope somebody reads it *and no doubt shoot the whole argument down in flames), but that's how i explain the lack of bsa twin-carb pre-unit twins
