This is a good question. I've always wondered this myself. When I co-washed for the first time ever a few days ago, I decided to turn the pre-poo into a DC on dry hair first, with heat for 30 minutes. My dc mix had a little olive oil mixed in, so it was pretty similar to what I normally do, before a wash. I then rinsed, and co-washed with a different conditioner. No shampoo, just water and conditioner. Stepped out of the shower, added leave-in, and a little serum, styled as usual. I let my hair air dry since I applied heat to it earlier for the DC. I really liked doing it this way; it saved time, and allowed me to condition my hair without using shampoo. I also didn't worry about rollersetting, going under the dryer, or silk wrapping my hair after the co-wash. I can see where co-washing can be done easily between shampoos, and if you DC and co-wash on the same day, you're killing two birds with one stone.