A more thorough answer to OP's question: If you run two pumps in series you will double(ish) pressure at the outlet of the second pump, but not necessarily see much of a change in spray ball action. Running the pumps in parallel will not increase pressure at pump heads, but it will increase flow at spray head, which can in turn give you more line pressure by default due to the spray ball now becoming a restriction because of the increase flow. But pressure will not increase over what a single pump can do on its own, just flow.
With two pumps, at least you always have a spare. If you go with two pumps, I suggest getting the exact model that you already have.