It appears to be the material. Here is a site where component materials are listed, including "mi-duvet":


page 5
... Canapé 2 places et fauteuil "Vinci". Boiserie cloutée main. dos mi-duvet et assise Bultex. recouvert en tissu Maïa. Canapé 3 places et fauteuil "Turner". ...
www.meubles-pascal.fr/htfr/0005.htm

And hereare a couple in English:


HEFEL TEXTIL AG - The ticking range
... The ticking used for quilts and comforter shells has to be able to ... FILLING. 68, Ticking Quilts, Pluckings and Half-Down. 78, Egyptian Cotton Ticking Quilts, ...
www.hefel.com/English/products/inlett.htm


PANTEX - SHEETS
... to-measure. Overlocked bedspread made-to-measure. Comforter. ... Goose feathers Duck feathers Capon feathers Half-down Polyether Polyester Empty casing in cotton, ...
perso.wanadoo.fr/pantex/aDrap.html

Whilst I would agree that "mi-duvet" is usually used to refer to the filling of a sleeping bag, quilt or whatever, the other half being something else, your description reads more to me as one of the transformation itself.

Is there any way of providing the sentence before and the sentence after the extract you have provided thus far ?

Nikki

I think it refers to a material.

Kika

It appears to me that the "jacket" part of the set is filled with down and the "sleeping bag" part is filled with a mix of down and feathers (mi-duvet). One attaches to the other in some way.