There are several types of these. Some are two coins split in half:




Note the edges of the coins, you can see the seam between the two coins.

On the next type, they cut the pieces of both coins to fit inside each other:

Note on this one you can see the seam near the rim of the reverse coin?
A second type they are made from two coins with parts of the two coins are turned with a lathe to make the two
pieces fit together:




On this one you can see the full dime fits into the back of the cent.
A third type is a hallowed out half with a quarter inserted on the reverse:


I heard that you can place the coin in a glass and bounce it off the sides to open these up. Most are heads on both sides, but the best ones is the tails and tails. Why? Because in a coin toss, heads is the most often called. Making your tails/tails the winner.