Depending on how you resolve those events you might eventually improve their relationship to the point of not triggering those type of events. You can prevent these events from occurring to those slaves by interacting(either meet or sex) with them every few days.
The simplest way to improve relationships between slaves is to have group sex and let them have to good time together(with you if you want). Once or twice will fix any relationship problems if you are good at sex interactions, otherwise it may take a couple more times.
Otherwise, your best option is to use the oblivion potion to erase one slave's memories and relationships.
You could also put one in jail and make the other a jailer, this dynamic also works for farm managers and slaves in the farm with good obedience or for headgirls and anyone not in jail or farm. But these methods are quite slow.