PHD (in the UK) have done a few down combis bag for a bit now. They're just standard ones cut a little bit wider/longer and there isn't any clear reason that a quilt should do the same job fairly well.
Their basic pitch for their main series is that you have that and a very light summer bag, use the very light bag in Summer, the combi in Spring/Autumn and the pair in winter. As well as the cost the whole thing is very pleasingly 'tidy' from a conceptual point of view. The 'cost' seems to be ~100g in Autumn/Spring (wider bag so heavier) and maybe 200-250g in winter. Seems a very reasonable trade off to me.
They also do a combi bag for their very warm series so with two data points can get a, very rough!, ball park figure for combination. The ultralight series is +5,-5 individually and -15 when combined. The warmer stuff is -9 and -6 going together to -33. These are rough figures of course, but they'll be honest estimates.
The combination rule seems to be something like combined difference from +16 :) That's with two bags fitting perfectly together of course. Even a quilt over a normal bag probably won't be quite as effective overall and two normal bags might be a lot worse.