A man and a woman cut their ties to everyone in this world and go into hiding. What can be said?

If they want to leave the world behind that is their business. Also if they reject the world and not the reverse then they aren't the outcasts. If you're afraid of them researching something extreme than nosing about their business will only increase their resolve and probably their pace of research. Let them alone and see if they grow as people. My bet is that at least one of them will and that will end the situation.