There are a couple of ways of doing this, although to be honest, using a second receiver is a better option in my opinion.
You can have both transmitters set to the same channel and only ever have one switched on at a time. This method will work with any type of radio mics but does require discipline to work.
The other option, which is somewhat unique to Line6, uses the fact that the receiver will lock on to the first transmission it hears and will completely ignore any subsequent transmitters on that same channel. When the first transmitter is switched off, the receiver will only then start receiving the second transmitter.
As I say though, being able to control things from the desk is always my preferred option; relying on the presenters to turn their mics on & off at the right times is a recipe for disaster.