What happens if one brother comes in under budget? What happens to the money left over?
You should be able to take your remaining budget and count it as profit.
If you come in under budget it won't help. The reasoning is that we should each be encouraged to experiment. One of us may go heavy on the art while the other focuses on marketing. Either way, there should be no punishment for trying as much as you can within the rules.
If one of us goes over, that will count against the final profit.