The Beiter String-Tool is a Multi-Purpose-Tool for many different tasks, not just for working on your strings. The Beiter String-Tool is sold in pairs because you can and should use it as a pair: you will get one String-Tool #1 (Black) 9,5mm and one String-Tool #2 (White) 12,7mm (1/2").
But you can choose to buy the String Tools as a single part together with the Tool Clip (part on top of the picture), which is the perfect storage device for the Beiter String-Tool.

With the Beiter String-Tool you can easily count the strands in your string and properly install your Peep-Sight. 
After you have pushed the first String-Tool through the string, you can count the single strands by inserting them into the grooves on the Tool. 
To be able to install the Peep-Sight easier, you can use the second Beiter String-Tool generating a bigger working-window.

The Beiter String-Tool is a Multi-Purpose-Tool for many different funtions not just for working on your strings. It can be used like a finger-nail!
Basically you can: 

• split a string for Peep-Sight installation;
• easily count the number of strands of your string;
• determine the middle of the string by counting the strands so ensuring that the Peep-Sight is in the center of your string;
• you can start a serving extremely simply and quickly;
• serve a Beiter nocking point more easily, defining the correct starting position;
• use the String-Tool as a lever to ease off a rest or glue from your bow!