Training without a partner is typically very limiting - many techniques are hard to simulate when practiced on your own. A single pell is a good solution for some techniques, but Liechtenauer's Art relies heavily on winding in the bind to attack the opponent behind the sword. Using two pells, one behind the other, is a great approximation of such a scenario - by treating the closer pell as a pseudo-bind, you can practice Liechtenauer's Zettel on your own.