There’s definitely some utility in separating roles— as an experienced designer and developer I can assure you that making the right design decision is a lot harder if it’s going to be a pain in the ass for you to code — but the most important factor is having the right expertise for the task. We’ve all seen the cargo culted loopdy loop code written by non-experts armed with tutorials, and having one of them do the design and one do the coding won’t work out any better. Same thing with devs and UI work. Design is a wholly different skill set that requires study and practice.