What do you think? Should sign up and log in links be split into two buttons or is it OK to have them in one?
N.B.: sign up form and log in form are on the same page.
UxMovement made a pretty interesting note about the subject - don't forget to also read the comments.
To summarize, it doesn't truly matter if you've got one or two buttons, the important fact is that the user can easily distinct both, especially through their labels: sign up and sign in are too similar, and even if they are two buttons far away from each other, users will mix them up.
That said, I'd stick to the standard and provide two buttons, as there are two actions. A new user won't click on a log in button, and a known user won't click on a sign up button either.