Think of it in the terms of SotN. Richter was fighting Dracula and was on the losing side. Maria, who's strength is equal to Richter's, gives her power to him and he uses that strength to defeat Drac. So yeah, they technically did it together.
This only happens when you actually lose the battle in the prologue. But since it's a prologue to the up-coming main game then you can't lose and that why Maria shows up and lends you her power. I think it's more for show rather then being canon-wise (and it does make for a flashy show). Richter handled Dracula on his own just like his forefathers before him.