Twin SU's on a 4cyl, 4 stroke engine are only ever feeding one cylinder at a time (alternately) anyway, and as 89mm explained, that constant delivery will probably work better with this type of carb than one veturi per cyl which would generate pressure fluctuations requiring a very rapid piston response.

That principle of feeding one cyl at a time changes a little at high engine speeds with long duration cams where inlet charge momentum can keep supplying air fuel charge into the cyl even after the piston has started moving back up on the compression stroke, but SU carbs simply won't run with genuinely big cams required to generate this effect, so there's little point really.

That's my thoughts...