You can add another teacher microphone by
Changing the second teacher microphone to Channel 2,
The HH/student microphone is on channel 2 as well,
If the teacher is using the second teacher microphone (has it on) the student mic will not work unless the teacher mic on Channel 2 is turned off.
If you use 2 HH/student mics and use the push to talk feature, then whichever microphone is activated first will have the audio and once the push to talk feature is off the other mic can be used.