If you have two puzzles and the boxes are the same length and width (height doesn't matter), then you can ship them together easily and save some money when shipping. Here is an example of two puzzles that I shipped a and how I did it.
Remember, no matter what put the puzzle pieces in ziplock bags! Notice that these are two different brands but the boxes are the same size in length and width. One is Buffalo and the other is White Mountain. The Buffalo is slightly taller but that doesn't matter, only the length and width so they can sit on top of each other. You will need to tape each box closed ON ALL FOUR SIDES for safe shipping.
Next this is to leave one puzzle face down on the table and then take the other and set it face up on the first one. Now tape the two puzzle boxes together on all four sides. Give the boxes a little squeeze so they are taped tightly. I like to do one side, then the opposite side, then the other two sides. This puts the tape in light tension.
Once you have all four sides taped together, you can see that the two boxes act like one. At this point, you wrap up the box just like you would a single puzzle. For wrapping instructions, see the article "How to pack your puzzle for shipping". By shipping in this way, you reduce the overall size of the two puzzles and that drastically reduces the cost. In case you are wondering, shipping this puzzle all the way across the country only cost $8.92 by purchasing a discounted shipping label on PirateShip.com. (Dec. 2, 2019) If shipped individually, these same puzzles would have cost $7.80 each. So I just saved $6.68 by shipping them together in this way. If these were larger boxes, this may not be as dramatic of a savings. These are both 8x8 boxes. One is 2 inches tall and the other is about 2 1/4 inches tall, but the height makes almost no difference in this case.