Wow, what temperatures are you planning to sleep outside in? Is the wool lightweight and puffy or is it dense? Most sleeping bags for extreme temperatures are very light weight and soft - check out a camping store. You want to make a sleeping that will dry quickly, it will get damp/wet from sweat, humidity or possible rain leaks AND should be dried daily.
I would suggest using ties instead of sewing. Remember the more air pockets, the warmer it will be and it will also not overheat you either.
To join the layers, use 2 zippers which meet or almost meet at the bottom middle, this allows the top and bottom layers to be totally separated - makes drying time faster because it is only half the thickness. This also allows if the bag is too warm for one of you, then you can unzip your side only. Most people make a double sleeping bag by joining two single sleeping bags - very easy to put together and take apart.
Use a woolen hat! Keeps your head warm and your body warm too.
If I was planning this adventure then my sleeping bag would be lighter than needed. If I was cold, I would cover the bag with additional blankets for warmth. I would NEVER be making sleeping bags. I would be purchasing two single sleeping bags with the appropriate temperature rating, and be making and/or buying the extra blankets. I have family members which camp during the Winter when it is -40F, and their biggest complaints are from being too warm or unable to dry their equipment.
I hope this helps.