"Combining degrees" can mean two things. It can mean just pursuing more than one major. For instance, you might double-major in math and physics. In multiple-majors, your majors are still separate, with their usual requirements. If you're double-majoring in math and physics, you have to do all of the work math majors do and all of the work physics majors do. I think most colleges allow double-majors, but some have stopped allowing triple-majors and higher. You usually need permission to pursue multiple majors.
Alternatively, two or more schools or departments (that's just what the divisions of the college are called, like the School of Arts or the Math Department) may get together and offer a special interdisciplinary program. For instance, if a college doesn't have a Logic Department, the Math, Computer Science, and Philosophy Departments might get together and offer a program in logic, combining courses from all three departments. In this case, you don't have to do all of the work of math, computer science, and philosophy majors; You have your own special requirements. It counts as one major and you earn one degree. For technical reasons, you usually enter one the participating departments and they award you your degree. These types of programs vary from place to place. I imagine several fields got their start this way. For instance, I wouldn't be surprised if cognitive science, which is now a full-fledged department at many schools, started out as a special interdisciplinary program offered by separate departments.
Edit: Woops, I forgot your other questions.
when can i combine , is it when i finish from a certain major or ?? should both degrees be related to each others , i mean math and something related to it let's say quantum physics or whatever ... does every university provide this system ... finally what is the advantages of combining?
I think the deadline at most schools for declaring a major is towards the end of your sophomore year. Since interdisciplinary programs count as one major, the same rules should apply. You should check with your school to be sure though. If you are pursuing multiple majors, you may be able to add on the extra major(s) at a later time; Check with your school.
Multiple majors don't have to be related; They can be whatever you like. There may be an advantage in choosing related majors in that the requirements usually overlap. For instance, Discrete Mathematics may be required for both math and computer science majors; You would only have to take the class once, and it would count towards both degrees.
