Take a gravity measurement before you do anything. That said, if you're relatively close to where you want your FG to be, your second pitch isn't going to add much character to the beer unless it's something super-wacky (e.g. Belgian Saison, et al.) and your fermentation temps are controlled.
Note that I keep clean fermenting dry yeast on-hand for just this sort of scenario. US-05 can be a life-saver in these situations. Buy a few packs --they store forever AND it's basically the Chico strain, so it makes a mighty fine beer when you're in a pinch for yeast anyway.