There are several reasons why you might want to have two disparate keyer types connected to your radio at the same time. Some us use the KX2 or KX3 for field day or have other reasons for want to have two keys connected at the same time. For example, if want to connect a computer key output (or a hand key or bug) to the KX2/3 and use an external paddle at the same time (non-Elecraft), you’ll need to build an adapter for the front panel Key2 connector. Below is a link with instructions and tips and how to build the adapter along with an offer for a free connector.
73, Walt W0CP