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Whenever you see "2(name of stitch)tog" in a pattern, it's another way of signifying that you are supposed to create a decrease stitch. In other words, you are going to create 1 stitch spanning over 2 previous stitches.
To perform a 2dctog, you want to think of it as starting the first dc but stopping before finishing it in order to start a second dc in the next st and then finish both stitches off together to create 1 stitch. Wrap the yarn around your hook 1 time, insert hook into first stitch and pull yarn through (you now have 3 loops on your hook). Yarn over and pull yarn through the first 2 loops on your hook. You should still have 2 loops remaining on your hook. Wrap the yarn around your hook 1 time and insert your hook into the next stitch. Yarn over and pull yarn through. You now have 4 loops on your hook. Yarn over and pull through first 2 loops on your hook. You should now have 3 loops on your hook - the original loop carried over from the last full stitch, the loop from the almost-done dc in the first stitch, and the loop from the almost-done second stitch. Now, do one last yarn over and pull yarn through all 3 loops on your hook. You have completed a 2dctog.
This technique can be used at the beginning of a project also -- for example, if you are working in the round, just insert your hook into the loop or circle that you have created to start the project and follow the instructions above as you work your way around your center loop. You will end up with a cylinder rather than a flat circle.