many years ago I was given a pair of duvets as a wedding present they fix together with press studs all along their edges- Meant you could use the whole as a 15 tog, or when it was warmer , use a single 10 or even 5 in summer. It was a long time ago but I use two when I need to.
I find using a 100% Cotton duvet cover on your duvet also keeps you warmer than a polycotton. We have a 7.5 tog feather duvet from George and when I wash my cover and use polycotton for a night while normal 1 drys I find it much colder same as bed sheet. (Hope this helps anyone)