@steph_longdon Thanks for reaching out!
While both styles of pants are designed for moving over snow in cold temperatures, there are some very stark performance differences that come in to play if you are thinking about using one pant for both activities. Let's break down what each style of pants are designed to do:
Nordic ski pants are designed to allow movement (normally utilizing stretch material for a snug but flexible fit), protect you from the elements, be somewhat aerodynamic, and, most important to this discussion, help you regulate your heat in an activity that requires a good amount of effort and fitness. While you can find nordic ski pants that are waterproof, typically you'll see a windproof layer (sometimes only in the front) and some sort of stretch and wicking material in the back to allow for breathability.
Downhill ski pants are designed to allow for movement (typically by having a relaxed, more loose fitting style), provide waterproof protection from snow and rain, and provide insulation and warmth for an activity that can generate warmth when skiing but also needs to keep you warm when sitting on a lift or standing in line. Ski pants will often also come with options specific to downhill skiing like belt loops, large pockets, suspenders, bib styles, reinforcement on the ankles to protect from ski edges, etc. Additionally, nordic ski pants are designed to provide ease of movement in an aerobic activity where downhill ski pants are designed with more durability and toughness for an activity where falling and scraping on rough snow is more common.
Ultimately, it is up to you to make the determination whether nordic ski pants would work for you while downhill skiing. The pants for each activity are designed to handle very different conditions, so you will want to consider if the nordic pants you are considering using will be able to hold up to the conditions you will face while downhill skiing.
Hopefully this helps, thanks!