Is it reasonable to ask a painting contractor to paint the higher levels of my house, while I paint the lower levels?
I think it is. Ask when you first contact the painting contractor and if they are willing to split up a job like this. If not then they won't need to come out and take a look at your house.
Some potential problems could be; No warranty offered since the contractor only painted certain areas. Are there good stopping points on your home? If not, then it might be difficult for the contractor to stop just at a given level.
Discuss this with the painter and find out how it would work. Also, you might be surprised that having the rest of the house painted at the same time might be a reasonable cost.