There is actually a feature available in the Family Grouping called "babysitter," which ensures that members of a family will not be scheduled at the same time as one another. To add this to a family, go to the Family Groupings pane and click on the family in question. Click the Edit Family button and then click on one of the two members of the family and check the "Babysitter" box. Then do the same for the other family member. Now MSP will never schedule both members of the family at the same Mass.