Two colours can only be applied one after another in separate days. It will be proibably more convenient if sanding of the stairs is done first and the floor men or your builders / decorators are happy to go around 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 paint is slower drying it will make sence to be the last to apply as it can be done by one person only over 2 morning for example.