I wonder if the non-double knit is a stable knit? If so that means it’s not really that stretchy while a double knit will stretch. For something like a jacket, it should be ok! I would just want to make sure they’re a similar weight so they’ll have a similar drape. (A lot of times, online fabric stores will send you a swatch so you can see it before purchasing.) I would only be concerned if you’re making a seam that requires stretching and one fabric will stretch and the other will not stretch, therefore your seam will not stretch either. But for a jacket, you probably won’t have to worry about this. 🙂