No need to oversize the winter tq. Order it so it fits you, then put your 20° on top of that (providing your 20° bag has a button foot box that you can open up).
2 reasons for that:
- You will have the best flexibility for the lowest weight and volume on your gear.
- You want to keep your winter tq as fluffy as possible for as long as possible. Water/condensation will slowly degrade your quilt, and with time make the insulation material collapse/lose it's insulation properties (unless you can dry out your quilt somehow/-where). If you put the 20° quilt on top of the winter tq (not the other way round), any possible condensation inside a quilt will be in the 20° one (the topmost one). It'll give you longer time staying toasty in the outdoors.