You have the capability of ensuring that certain subjects or certain groups will not be taught on the same day. By utilising the Exclusions tab on the Subjects or Groups screen, you check the appropriate groups or subjects that are mutually exclusive.
The result after solving:
As can be seen, 8a does not appear on any of the days when 8c,d,e, and f occur. Since we never checked 8b, it can be placed on the same day as 8a.