The stacking characters are actually alignment characters. The "back" character is really a centering mark, and the "front" character is a right-align mark. If you use just one, you'll see this effect. When you use both, what happens is the text gets centered, and then the text after the right-align mark gets put over to the right. You then have to add a lot of spaces at the end to push it back over on top of the original text.
In fact, you don't even have to use the first centering character! You can type the word you want, and then a right-align mark, the word again, and a lot of spaces. The number of spaces depends on how long your text is, of course. To do really fine alignment, you can insert an invisible character, then replace a space with a period or a curly brace ({). Spaces are 4 pixels each, periods are 3, and braces are 5. So if you want your (front) text one more pixel to the left, replace a space with a brace. One pixel to the right, replace a space with a period. You can then use combinations of these to get 2 and 3 pixel alignments as well.