I'd argue yes, but not necessarily on the strict terms I think you're implying... There's no reason why you can't make a single bag with the same insulating capacity as two other bags, but the two-bag system has a big advantage in terms of flexibility. I have a 1-2 season down bag which is great when it's not too cold, and a 3-season synthetic bag which I use when it gets too cold. Now, I don't go out in winter any more, but I could put those two together for to produce pretty good 4-5 season setup - so I get 3 different setups (suitable for anything from balmy summer to sub-arctic winter) but from only 2 bags instead of 3.