I think what you’re describing is a “duvet cover”. A “duvet” is a two part comforter with a bulky padded insert and a thin washable outer shell. There are a variety of types of duvet covers available in all kinds of different materials and many of them would essentially match your description of “two sheets sewn together” but they can also be significantly thicker and heavier weight materials, they still would be thin and won’t have the padding of a comforter though as that would be what the insert is normally for. The giveaway would be that there’s usually a zipper or button flap somewhere to insert the comforter insert into, and there are usually buttons or ties on the inside as well to keep the insert from bunching up.

You could just get one of those and skip the insert. I often sleep with just a duvet cover during the summer.