I think separating sound design and composition is a very good practice if you can keep a strict timetable.

I usually mix the two together to keep things interesting for myself, but the other night I focused on composition first in order to get all my ideas out before going to work, then sound design came after. I completed a rough draft in an hour (felt like it went by faster than that though). Splitting the two helped me focus a lot more, but chances are I'll keep mixing them since I'm so used to it lol