When I did this one ( http://cakecentral.com/gallery/1925293 ) I rolled out a piece of each color, then cut one edge of each in a straight line, then overlapped them about a quarter inch with a tiny bit of water (I used a narrow paintbrush and did just the overlapped edge), then rolled over the join till it bonded together (a small amount of cornstarch or PS will absorb any water that squeezes out when you roll, but if you use a minimum of water it shouldn't squeeze out at all). If you're covering the join with a ribbon (as in the picture you linked) or other decoration (as in the flowers I used), it doesn't need to be a perfect line. You just have to make sure that the two colors are well bonded before you lift it to place it on the cake. Getting the line exactly where you want it to be could be a bit tricky, but if you plan ahead it should work out well. Of course, mine was only a single layer six inch, but the principle should be the same for any size and shape. HTH
Thanks cakeyouverymuch. Your cake is beautiful so I'll try your technique!