Piggybacking is simply sticking another magnet onto the fixed magnets of the rest. On the dual magnet version there is a small recess that holds them securely in place. You do not need to worry about them falling out or getting lost either. They actually can be quite tricky to remove once piggybacked but doable with a little practice. The single magnet version has a longer 1/4″ magnet in place so the piggybacked magnet has no recess for retention. If you piggyback a magnet there you may want to go with a longer magnet instead or put a drop of superglue to reinforce the joint.
So, why would you want to piggyback? The most obvious reason is to remove “sag” at brace with heavier arrows or point weight. Some, that prefer the lightest drop possible utilize a little bit of sag without detriment. Beyond that strengthening the magnetic hold can also lessen the need for a low nocking point and improve guidance. It can be fun to experiment and observe changes in grouping, crawl, etc. With a perfect tune the strength of the arm matters very little and, conversely, with a poor tune if small changes to the rest fixes it, that probably confirms the marginal tune. For maximum forgiveness and range of adaptability in stringwalking our bow and arrow combination needs to be optimized as the first step always.
So how is magnetic strength affected by piggybacking. First of all, the magnets diameter affects strength more than length. Once the length bypasses the diameter the returns become drastically less. The rest magnets are 1/4″ diameter so by the book after 1/4″ of piggybacking it is not doing much. In practice though piggybacking up to 3/8″ has notable effect. A single magnet 1/4×1/4 is stronger than two 1/4×1/8″ magnets piggybacked. Piggybacking magnets in opposite positions on the rest results in more strength than piggybacking those two magnets on one position only. Asymmetric piggybacking is just fine and has no downside over “balanced” piggybacking if it results in the strength you are looking for. Just think about the forces being additive to the rotation of the arm.