Adding a little bit of shampoo to the conditioner
 

I've been wondering about this too. Cowashing mostly works for me but sometimes my hair looks flat on top... I've been considering adding the teeniest bit of my low-poo to my co-wash, so it would be mostly YTC with a smidge of Renpure Organics.

I imagine this would work, but hopefully someone else chimes in!

I've done this and it works. I actually read about it on here, I think it was called a "co-poo". I would use more conditioner than shampoo. I would just mix them in my palm and use that to scrub my scalp. The consistency was a bit watery but it got my hair and scalp clean.

I agree with the OP that you can do that. However, you may find that doing a brown sugar scrub would be most effective for a film of buildup on your scalp. Just add a tablespoon of brown sugar to your co-wash conditioner, then give your scalp a really good massage.