Yes you can do it with 2 sheets.You will however have to put either snaps,or ties on the inside of the cover as well as the quilt to keep it from shifting inside the cover.You can finish off either the top or bottom with velcro or buttons and buttonholes,i prefer the latter.Or you can use co-ordinating ties.HTH
Kathi

Whenever I make duvet covers I always sew a piece of twill tape or ribbon into each corner of the cover and sew corresponding ties on to the corners of the duvet. Then I tie the cover and duvet with a double knot. It always stays secure until I want to remove the cover.
If you use the top edge of the sheets for your open end you have a nicely hemmed opening.

Duvets are used a lot in the UK. We make the covers using those strip snaps on the opening or buttons, velcro can be a problem if you should get the duvet upside down and it catches on your face !!! Some makes are open on the sides but I don't care for those although they are easier to stuff the duvet in.
We don't put anything on the inside to hold the duvet in place. If you get the cover on correctly and the duvet right into the corners they seem to stay in place.
I have never had any with ties inside and we have been using them for about 30 years.

Okay! Thanks for so many responses! I will experiment this weekend. I am a novice sewer so this should be fun!

I'm with Shirl. We never attach the duvets to the interior of the cover. Are our duvets "fatter" than others? I can't see why you'd need to add ties. French duvet covers are made a little differently. The bottom is cut so that it fits the duvet from the outer edges towards the middle for about a third of the way on each side, then the middle third is left longer. (Imagine a sort of tube you stuff the duvet through).This tucks under the mattress and means you don't have to add buttons etc. It holds the duvet in place on the bed so it doesn't "travel" while you sleep. It looks very tidy. Hope you understand what I mean!

I think european duvets are fatter. I think Rheba is looking to make a cover for her nice quilt not what Islay and I would call a duvet. I think "proper" duvets are coming into the US but from what I have seen they are mostly feather ones. I have shipped out duvets from England (and covers - Bob the Builder and My Little Pony amongst others) for my daughter and children in California. When my daughter went out to college there she took her Marks and Spencer duvet with her!!!

In Canada and, possibly the US, what we call duvets are filled with a synthetic filling ,a feather mixture or pure eiderdowwn and the price varies accordingly.There was a thread on duvets some time ago and I think one of the differences between Canadians and Europeans is that we use a sheet with a duvet over the top and thus the cover doesn't need to be removed and washed as often. I recall Islay in that thread saying they just used the duvet itself. I have had a duvet for many years and have made several covers for it, always with ties, probably because sometime suggested it but I'm sure not using ties works just as well .In the summer I take the duvet out and just use the cover on the bed. Whatever and however, they are certainly the cosiest bedcoverings I know of.

Yes, I think that there is some discrepancy between what we call a duvet in America and what it may be called in Europe. What we are calling a duvet, is, usually a comforter.
I think that what Europeans call a duvet is what we might call a feather bed.
So, we buy a "duvet cover" and buy a "comforter" to put inside it. Go figure!
Anyone please feel free to correct me if they know different.
The difference is that the comforter is more of a quilt and the feather bed is big and fluffy, usually filled with down.

I have made the covers,and I do like having the ties to help keep it in place. Maybe it depends on how much you move around and how fluffy is whatever that you put into the cover.

Yes, you're right. european duvets are fluffy things - most people use those filled with polyester for easier washing.