This is the first step, filling two bags with the colors you want to use. These are 18" plastic icing bags, but you could do the same thing with smaller bags.
Step 2. Insert both bags into a larger bag that has been fitted with a cupcake tip. I'm not sure if you can buy these on the street. Ours come from Bakery Craft. This bag is 25 inches.
The trick is having enough strength in your hands to squeeze the bag so that both colors come out at an even rate. I move my swirl in a counter clockwise direction beginning at the top on the outside of the cupcake.
I made five dozen swirled cupcakes. I had to refill my bags once. The first two dozen are the easiest. I need to work on refilling the bags, etc. After a while, the bags start to settle too much into the tip, making it very hard to get the icing to come out. And after about four dozen, my hands and arms are worn out from applying all the extra pressure.
All in all, they look impressive in the cake case, and gets attention. That is the main reason to do it. It doesn't make them taste any different!