Technically, you are only guaranteed a room that accommodates 2 people. But, many people do book through priceline for a family. We have booked a number of times through priceline and we travel with our son (3 total in room). In the Orlando area you are very likely going to be able to receive a room with 2 double beds. If the hotel can only give you a room with a king, you can get a roll-away (probably at an addtional charge).
I just recently booked through Priceline for 2 rooms and it is very easy. When you go on to Priceline and start inputting your information, there is a place to select the number of rooms you are requesting. You definitely want to pick 2 rooms at this point and you will receive them at the same hotel. If you were to book one room and then put in a request for another room separately, yes, there is the possibility that you could end up at two different hotels -- that's why you put it through as one request for two rooms.
When I booked the two rooms, it was for my family and my sister's family. And since we were not arriving at the same time, I was able to list her name on one of the rooms and mine on the other.
If you haven't bid on Priceline before, I recommend BiddingForTravel.com This is a website where people post at what rate they have received rooms. You can go to Orlando (or wherever you want to bid on) and see what hotels people are receiving and at what rate. Also they have a FAQ that explains how to bid. After reading the FAQ, you can use a form to request assistance or a bidding strategy.
