When picking out necklaces to wear, you usually find one, lay it around your neck, and hook it in the back. Then you let it fall and adjust as needed. Instead of following this process with multiple necklaces, it's actually better to take a different approach entirely.
It's normal to wonder if you are wearing your necklace at the right length. So, when you find two necklaces around the same length that you want to layer together with an outfit, start by placing them around the back of your neck so that the clasps are dangling in the front of your body on the sides of your chest. Instead of clasping each necklace together like normal, connect the opposite sides of the different chains. Therefore, the necklace clasp on the right connects to a different necklace on the left and vice versa. You're actually creating one long connected chain around your neck that can be pulled to create the look of two different-length pieces.
Once all four loose ends are connected, you can twist the necklaces back around so the clasps are at the back of your neck and the jewelry is visible in the front. Now, you can have fun with how much or how little you choose to layer these pieces. The more you pull the necklace on the bottom, the tighter the top necklace will get around your neck. Similarly, if you make the top necklace looser, it can be layered just above the bottom necklace with less of a gap between them.