Two car driveways are generally between 20 and 24 feet wide. While this does depend on several factors, you can expect most to measure around there. Since most cars are six feet wide, you should be able to comfortably park both cars side by side, with room to spare.
Although it may seem silly, consider bringing a tape measure with you to showings. Since a two car driveway is important to you, you want to make sure your new home meets your needs!