Two colours can only be applied one after another on separate days. It will be probably more convenient if stairs sanding is done first and the floor men or your builders/decorators are happy to go around and apply the paint on the risers.
You have to consider, that regardless of which colour is applied second, the protection of the first one is of importance here. A good amount of extra time is required to apply low tack masking tape around every step and riser to protect the already finished area.
As the paint is slower drying it will make sense to be the last to apply as it can be done by one person only over 2 mornings for example.