Or they could just,

1 Healer in party regardless of class/job - MND bonus is active
2 Healers in party regardless of class/jobs - MND and PIE bonus is active

if SE determined thats to simple and there needs to be a technicality deter double stacking of class/jobs then;

2 Healers in party of the same class/job - MND bonus only
2 Healers in party of different class/jobs regardless of which class/jobs - MND and PIE bonus active

any composition is now viable bonus wise for raid duties but your still encouraged to bring 2 different healer types.
the more lax 4-man duty environment now gets the MND bonus uniformally

alternatively they could leave it as is for 4-man duties and only implement the change to 8-man's, but im not sure how technical it would be to implement that/viable