I would not recommend trying to solder wires to the USB pins on a Teensy - way too fiddly, and would be prone to breaking off! Instead, buy or make a short adapter cable, and plug it in. If making one, the simplest way is to buy a mini USB cable and cut it, then soldering the wires to whatever connector you choose for the external connection.
But, if you want to use it with two keyboards, why not just case it and use it as an external adapter?