Arne, I was at first not convinced by your argument, since the two force vectors arising from the asymmetry of the two canted keels are what they are, however you care to resolve them, and they sum to a single vector acting approximately downwards (in the direction of the hull's vertical centre line.)
You can rotate the diagram, and resolve the individual forces any way you like, this does not change the above statement.
However, I thank you for forcing me to think a little more deeply on the question, and after “wasting” half a morning I now realise that at an angle of heel, this resultant “downward” force (along the hull centre line) does resolve into a useful horizontal component in the direction of windward, and a component vertically downward which might add to the righting moment.
The lift from a single fin keel, on the other hand, is perpendicular to the keel, and at an angle of heel this resolves into a horizontal component in the direction of windward and a component vertically upward (along the hull centre line) which might subtract from the righting moment.
The numbers will be quite different and there is still the question of drag and, of course, many other design factors, so what this small theoretical discussion means in practice probably does not amount to much. I still think the double-advantage claim of twin keels (that one keel is pushing to windward while the other is simultaneously helping to keep the boat upright) is fallacious if for no other reason than that is an over-simplification, but I do finally understand now that my analysis was also incorrect.
What we know in practice is that apart from the other advantages and disadvantages of twin keels, this configuration can, as David has informed us, be quite comparable with a single fin keel both in terms of windward ability, and the ability to stand up to sail.
By the way,
I don’t think anyone has answered the last part of Raymond’s question “… lots of rolling downwind with bilge keel?” I am given to understand that rolling is not induced by having more than one keel, and would have thought that bilge keels might actually damp down any tendency to roll. Again, this is probably a somewhat spurious over-simplification, calling for an answer based on practical experience.