How do you use a screwless terminal block?
To add to what Leon said, look at the opposite side of the connector to where the pieces of plastic are sticking out.  There are probably grooves or slots there intended to mate with the plastic tabs on another connector.  This is what is called stackable.  You may have to slide two pieces together to get them to end up connected flush with each other.  They may pop together, but that is more unusual.

The two pins are not for better electrical connection, but for a better mechanical connection.  Solder is a very soft metal, and a single solder joint could be overstressed by the force of pushing on the spring tabs.  Two separate pins spreads the force and also descreases torque on the joints.  Make the hole in the PCB only a little larger than the pin diameter to minimize the amount of solder between the harder and stronger materials of the pin and the PCB hole.  I would make the hole just big enough to guarantee the pin will always fit given the worst case dimensions and PCB hole diameter error.