If you ask someone whether he'd rather kill two flies or one human,
probably everyone will choose two flies.
I should start by noting that flies don't have pain receptors, so let's change the question to "two pigs or one human".
The utilitarian answer is still to kill two pigs, because the utility from a human (to all "moral agents", as philosophers like to call them) is greater than the utility from two pigs.
It's just like a question: would you kill a (or say, your favourite) political leader or a common fellow on the street? Obviously, the work of the political leader affects a lot more people than that of the common fellow's.
In addition to that, pigs are generally considered to be less intelligent than humans, thus are capable of feeling less "intellectual pleasures" (according to Mill an intellectual pleasure is intrinsically superior to a sensual one, while according to Bentham the superiority comes from the longer duration of pleasure - both arguments are effectively the same).
However, when it's "pig's life vs. human's tastebuds", the utilitarian response, I think, is to save the pig's life because we're directly comparing sensual pleasures here - i.e. animal slaughter and meat consumption are generally not good things in utilitarianism.