As far as I know, you can only accept one offer. You can only attend one program, and it would be unfair to the people who are wait-listed if you accept more than one offer. Also, once you accept an offer and pay the deposit, you are officially given a spot and they prepare you for matriculation. I don't know all that goes on internally, but if you are filling two spots, it is very disrespectful to the program you don't attend, as they spent all that time preparing you for matriculation and you are essentially ditching them. Not to mention you will lose one of your deposits.
There are some circumstances where you realize late in the game that you have to accept an offer at another institution after you've already reserved a spot elsewhere (family emergency, job offer, etc). These are rare circumstances in which it is okay to accept more than one offer as long as you explain your situation to the first school so they can cease the matriculation process and take someone off the wait-list. I would advise against abusing this, as I mentioned earlier, it is very disrespectful to both the program and the people on the wait-list.