In re: the depictions in the Maciejowski, some observations. I've just done a flip through the giant coffee-tabled edition of the same ("Old Testament Miniatures," publised by George Brazilier). There are a couple of images that are quite clearly, to my eye, showing the body as separate from the sleeves, much less ambiguously than the picture that Ernst posted above (the chap with the three-color war hat). Below, I've linked the scans on the Morgan Library site of the ones I mean.However, the gambeson draped over the shoulder shows no sign of the doubled shoulder line. There is certain evidence of gambesons being worn over aketons at the end of the century, but it's worth noting that nowhere in the Maciejowski are quilted armors with two colors shown, for example, no blue sleeveless gambeson over a red aketon appears. The sleeves always match the color of the body. If all of the doubled lines at the shoulder indicate two pieces, they were very conscientous to always wear red with red, or white with white. I think the evidence leans towards it being a single garment with a noticeable sleeve seam.
If I may give some insight as a professional costumer: this does not necessarily indicate a sleeveless quilted garment worn over an entirely separate, sleeved quilted garment. It seems possible to have the sleeves be made up separately and then set in deeply into the armseye of a thick-bodied vest to give the appearance of two items, but with the intent of allowing the "internal" armseye where the sleeve attaches a fuller range of movement, while allowing the "external" armseye to be as heavily padded as the rest of the torso and thus not compromising the protection in the area of the armseye seam. It could also involve quilted sleeves worn on a sort of thin arming vest beneath the heavy separate body. I used to work in the costume shop of a professional ballet company, and we had military jackets for our guys dancing the "Nutcracker" that involved a stiff sleeveless vest and a separate spandex undergarment with sleeves that matched the vest. When worn together, they gave the appearance of a single jacket, but the dancer had much greater freedom of arm motion than he would have had with the jacket and sleeves made as one.
In this period, there are three broad styles of sleeves on quilted coats: half sleeves that end above the elbow (Maciejowski, Huelgas Apocalypse), full length narrow sleeves (most of the other MS), and full length sleeves with integral mittens (Maciejowski Bible, Bussy-St. Martin sleeve).