Do I need 2 separate applications in order to apply to 2 separate phd programs of a university?

Lets say i am applying to phd programs of uc davis and i applied to both "school psychology" and "development psychology" programs that they have. Do i need 2 applications? Do i pay double of the application fee? Thanks.

That kind of policy is determined by each University. You need to contact the registrar at U. C. Davis (or wherever) and just ask them. If the two different Ph.D. programs are both administered by the same department/faculty and applications go through the same admissions committee, they might let you get away with only one application.

But I would think twice about combining applications for a doctoral level degree, even if the procedures allow this. Admissions committees are usually very selective at the doctoral level, and there is a lot of competition. One of the things they look for in applicants is motivation/commitment. I think it might be perceived as a negative factor that works against you in the application process if it looks like you are unsure of your direction. Saving an application fee would be very penny-wise and pound foolish if the resulting mixed signals brand you a flaky/undesireable candidate and get you turned down from both programs, right? Far better to do some homework about the programs in advance, invest some time in trying to meet and speak with a couple of faculty members in each track (even better if a professor you currently know can make an introduction), and then make a clear choice of which track you want to apply for. They will respect you for doing your homework, and then having the faculty contacts might help you gain acceptance into the program in subtle ways as well.

I would contact the school before submitting dual applications. Also, if you're applying for a doctorate program, don't you think you should write properly?