I recently got accepted into two universities for the Fall 2013, and both of them asked for me to submit the enrollment intent. I submitted the intent for one of them but then one of my friends said that since I did that, it means I HAVE to go there. But I am still deciding where I want to go. The other university has asked me to send in the form within 2 weeks otherwise I would loose a seat. So let's say if I send an intent to them too, would I be able to pick which one I want to attend when I make my final decision in a few weeks?
Universities typically consider such intent statements to be binding.
If you submit these statements to multiple universities, they will all hold seats for you, seats which could otherwise go to someone from their waiting list. This hurts those other applicants. It is also a serious inconvenience to the department's admissions committee, as they need to meet their enrollment targets as closely as possible; failure to do so can affect their funding, and in extreme cases, the survival of the graduate program. So this is at least inconsiderate on your part, and arguably unethical.
If the institutions find out that you have done this, it is possible that they may rescind their acceptances; this is uncommon but has been known to happen. In addition, you would certainly forfeit any deposits that you paid.
At the undergraduate level, where deposits are almost always required (in the US), this practice is called "double depositing". The National Association for College Admission Counseling, in their "Guide to the College Admission Process", says:
Colleges view dual or multiple deposits by students as serious violations of trust and may revoke a previous offer of admission from any student who is found to have sent tuition deposits to more than one college.
My advice is this: at this point, you should plan on attending the university X to which you've announced your intent. However, if you are absolutely convinced that doing so would be a huge mistake (not merely that you think Y is a little bit better), then you could contact the admissions committee at X, explain your reasons, apologize, and ask to be released from your agreement. If you do this, you should do it as soon as possible (ideally within days - don't stew about it for two weeks), and know that it will probably burn your bridges with X, as well as forfeiting your deposit. If X says it's all right (they probably will - if you don't want to be there, they don't want you there), then you could send your intent to Y.
Some people might argue that you are also ethically obligated to inform Y of the situation. I think it's kind of a gray area, myself.
You might also discuss the matter with an advisor or other professor at your current institution. They might offer some perspective on X that would help reassure you. In general, most people say that it isn't so crucial which institution you attend, within reason - your own skills and motivation are a much greater factor in your success.