What are you trying to accomplish with these features? I bought a softshell jacket and while I was impressed with its ability to shed water or snow, it is heavy for the warmth that it provides. Unless you use waterproof zippers and seal the stitching holes, the pants will leak around the zippers. Also, air can leak around unprotected zippers. I have some fleece pants that surprised me at how cold I felt around the full-leg zippers. Please consider thoroughly these features/drawbacks.

Is the double fabric for reinforcement against wear? You may be happier with adding an outer layer of some nylon fabric, from Cordura to a lighter one. The ankle area is not a fabric wear concern for me, but I am not hiking (or skiing/snowshoeing) in the same conditions as you are. For my purposes, I would want to reinforce the anterior (front)knee and the seat. Most solid, wind and water-resistant nylon materials would help with wear and repel wet should I find the need to sit or kneel (intentional or otherwise). Another fabric is unlikely to stretch the same way a softshell does, so account for some ease as you match up those items. I am ABLE to sew, but often find that it is cheaper and more practical to but ready-made on-sale, perhaps modifying as you indicate. Separating zippers are quite expensive, so you if you still intend to insert a full-length separating zipper in each outer leg, you might look for a used pants in a thrift shop planning to "harvest" those zippers. It is unlikely that you would find waterproof ones from this source, so plan on adding a strip of material with its own side closures to keep wind, snow, and rain out of the zippers. It sounds to me like more work that I want to do. I'd go with water a and wind proof light shell pants. HYOH, etc., of course. Good Luck!