Keeping two people apart who are part of the same family can be done with the babysitter option. However, keeping two people apart who are NOT part of the same family is a little trickier. There are two options available.
The first is to create alternating can't serve dates (i.e. Minister X can't serve every other weekend starting with Aug. 3rd and Minister Y can't serve every other weekend starting with Aug. 10th).
The other option is to make a "mock" family for the two ministers. This would enable you to use the babysitter option even though the ministers are not really in one family. Nobody would know that they have been put in a "mock" family together -- except when navigating within MSP. The only way people would see that they have been made part of a family is if you export a list of just families. In this case, you could manually delete this "mock" family from the list.