I divide cowslips now or after flwering. Just pull apart and back in the ground or, preferably for me, into pots.
Dahlia's often pull apart satisfactorily for me too but, as already said, each tuber must have a stem. I simply pot on into dry compost at moment gradually damping as growth emerges. Not too keen on cutting tubers unless I have to though. I like to wait until my tubers are quite large before I consider propagation