Replying to a university offer
It is not wrong to request for more time to make your decision. But, as @Nate pointed out, you may not always be granted what you request for. 

If you do not receive the time you ask for, then it is a gamble you may choose to make or decide to play it safe. Many go through this sort of situation. 

There is however another option. If you can't postpone your decision for the first institution, you could try preponing the your offer result of the other institution. Give a kind request to know your status of the application to the other institution to be in a position to decide earlier. This is also just a request, however it is still worth a try.