Edward, that's very kind of you but alas I think you are right that the cable won't be long enough, unless of course on the original Alfa set up the cable was not one single cable, but more than one cable that joined together with a nipple of some kind. I'm still finding it hard to picture how Alfa did it.
Here's what I've been told:
Alfa Romeo connected both choke levers together on the twin-carb boxer engine. If you look at the carbs you will find that the lever works in the same direction on both carbs, so one is always backwards.
If you want to connect just one, as you did before, simply connect up the the carb that "pulls" rather than "pushes".
What Alfa did was to clamp a cable guide to the throttle balance bar and loop it around to the second carb. The choke cable inner was clamped to the choke lever on the first carb, then looped through the cable guide to the second carb choke lever. As long as the cable guide is firmly attached to the balance bar assembly, pulling one choke lever will also pull the second one as well.
This technique effectively pulls both choke levers against spring pressure, so you don't have to use a solid inner choke cable - the more flexible Bowden cable can be used instead (the ones that can be locked by a quarter-turn of the choke control).
You need a cable that is long enough to pass from the choke position in the cockpit (centre tunnel?) through to the first carb and then loop around and back to the second carb.