This is not simply unfortunate, it is professionally totally unacceptable. Whether or not they seek to pursue their grievances, you should consider taking disciplinary action against either or both, on the grounds that it is unprofessional to behave in this manner in front of pupils. For a first offence, an oral warning would be appropriate. You should also remind them that they have a duty, as professional colleagues, to work together. If they wish to pursue their grievances, you should see that the school's grievance procedure is followed, commencing with the offer of an informal resolution, with other stages following if necessary.
CAN you clarify the question of passing through the threshold this year, in the light of the new compressed main teachers' pay scale?
All teachers currently on points 7, 8 or 9 of the current main pay scale, or who are placed on M6 on the new scale from this month, are eligible to apply for threshold assessment. If they are successful, those who were on point 9 in the current year will move to point 1 of the upper pay scale, backdated to September 1 this year. Those who are currently on points 7 or 8, or placed on M6 on or after September 1 this year, will have to wait another year, moving to point 1 of the upper pay scale from September 1, next year.