In order for a staff member to have a separate email address in Schoolcomms as a staff member vs. as a parent, they would need to have two separate records in SIMS (i.e. their parental record in Section 5 of the child's record being independent of their staff record).
By default, SIMS typically links the two together - so when you change an email address on the parental record in Section 5 for example, it would also update the email address on their staff record. The resolution in this case is to unlink the two records in SIMS, so that they are treated separately - the best way to achieve this would be to remove the parent from section 5 of the child's SIMS record, and then re-add them again, ensuring you click on New when the matched people box appears.
This will in turn mean that Schoolcomms will treat them as separate contacts, meaning they will have a separate email address as a staff member vs. as a parent.