Indeed, I can see no difference in progression, be it a 2- or a 3-couple
dance, or a 3- or ac 4-couple set. In SCD, you progress one place at a
time, except in some 5-cp dances, but even these are never referred to as
having "double progression".
The advantage of keeping to 4-cp sets is that you get to dance with
different faces each time through, whereas, when you have only 3 cps, the
scenery is not so varied.
Back to vocabulary; is "sigue" used this side of the Atlantic?