Single and Plied

What happens if you mix single plied yarn and a multi plied yarn in a project? I know they shouldn’t be mixed, but why? I have a single ply I want to use as a lovely sweater and there is a multi ply that would make a great color work punch. I don’t want to do this if it won’t turn out well.

Swatch and see how you like the two yarns together! Single-ply and multi-ply yarns can behave a bit differently/look a bit different, but for me that’s not necessarily a dealbreaker, any more than using a smooth 3-ply and a fuzzy laceweight mohair would be a dealbreaker for the right project.

With a sweater, in swatching I’d want to check the drape/bias of the single ply vs. the multi-plied yarn, since I suspect that would be where you’d notice a difference.