Accepting 2 postgraduate university offers
If the programs you are interested in have a limitation regarding how many new students they take in each term, you should probably not accept both offers. It could mean that someone else interested in a slot would potentially not be accepted into a program. There might be the possibility of extending the deadline, and it won't hurt to ask them.
I understand it's hard to decide on a program, but you should save others' time and effort by first deciding, then accepting. There is no hard requirement that I know of, however, that explicitely prohibits you from doing otherwise.

Make sure to make an informed decision, e.g. check the curriculum for both programs.