The correct answer to this open question is the following.
What I would do as a manager is the following. On one hand, I firmly follow the ethics and values guidelines and strongly invite the couple to stop seeing each other immediately because this is against the rules and politics of the organization and affect the activities, work, and results of the company.
On the other hand, I would have a conversation with these two members to allow expressing themselves before taking other kinds of actions. Once they have given me their motives, I would let them know the way their relationship risks the company and their businesses with other partners. I would let them know the way their behavior has affected both companies.
I would ask them to offer a sincere apology to their respective managers and team members and commit themselves to dedicate fully to their work.