maple syrup and brown sugar are going to add two very different flavors to your beer. I would recommend using one or the other to begin with. Secondly, make sure the syrup is pure and not corn syrup with maple flavor and if using pure maple syrup I would add when you are finished boiling so that you don't lose the flavor in the boil. Stir in right after you turn the flame off. Are you brewing an extract batch? If so, the brown sugar will help dry the beer out some, depending on the amount you use. Finally, this is just the tip of the information iceberg that lies beneath the surface of brewing knowledge. If you are just beginning, start with one ingredient, keep accurate notes and modify the recipe as you continue in your brewing.
One last thought, if you want maple flavor and are not intent and getting any sugar gravity, you could just add maple extract to the beer prior to bottling. This may be a better way for you to achieve the flavor you want. Just add to taste.