I am happy to say why it has happened in places where I have been in just such a situation.
Normally a diocesan priest has one day off each week. If there are two priests, that means there are two weekdays when the parish has only one priest. Depending upon the situation of the parish, one has to account also for funeral Masses. If it is a large parish or a predominantly older parish (or, worst case, both!) one can easily have more than one funeral per week and that has to be put into the equation…as do weddings, which sometimes happen on days other than Saturday, especially if again it is an active parish.
Also, the priests may be assigned to your parish but if they have some additional assignments, that has to be taken into account. One or both may have sacramental obligations elsewhere. If one, say, were a hospital chaplain, he may be needed to say Mass at the hospital. Or, at least, his schedule has to be taken into account in making the liturgical assignments.
Finally, we would coordinate with the deanery and the Vicar Forane so that nearby parishes were working in tandem. Parish X would have Mass in the morning, Parish Y at Noon and Parish Z in the evening and our parishioners would have more options rather than all of us having Mass at the same time.
It can seem straightforward…but it really is a problem when you have committed to have two Masses per day and one of the priests is sick or away and you end up with a funeral that can only be scheduled for that day – and the remaining priest has to cover three Masses, which requires a call to the chancery.
I would suggest you ask about the possibility of adding a Mass during the week on a day/evening which is neither of their days off and they can gauge the interest.