Whenever possible, catchers throw with a 4 seam grip. As has been mentioned, 2 seams is for pitchers and will give you better run on the ball, exactly what you don't want on a throw to a base.
It is easy to practice this at home in front of the TV. Just take your mitt and a ball and place the ball randomly in the mitt. Quickly bring your hands together like you would when transferring, and practice recognizing where the seams are and quickly moving them over to a 4 seam grip. Repeat it as often as you want. The more you do this simple drill, the better you'll become in a game when it matters and the easier it will be to get the proper grip while going through your throwing motion.