Shinobu and Tamayo, despite not knowing each other well, made an excellent pair regarding their abilities. Considering Shinobu killed demons using powerful Wisteria toxins - and Tamayo was able to rehabilitate demons with various potions.. they made a great pair..

"Shinobu, dear.. mix in some Wisteria toxins in it - we know it won't kill Muzan, but it should help leave the lasting effect of ageing on him.."

knowing this might be the revolution in regards to killing Muzan, they went all out.