You recently found out that two of your employees are having an affair with the added complication that they are both already taken and you have no idea how to handle the situation. Although relationships outside of the workplace should not impact internal relations, it will and is something that needs to be addressed and aired. Here are some suggestions on what you can do.
Moving teams/departments
If the two employees are working on projects together, within the same team or department or just simply having to work with each other, you could suggest a switch of roles for one of them. If there is a fall out in the future, then it could go seriously wrong and one person may end up leaving. If they are both valued employees that you do not want to lose, it is worth suggesting and encouraging. If they want their relationship to work, they should be open to it as well.
Discretion
Have a meeting with the parties involved to share how you feel about how they should be acting within the workplace. Suggesting full discretion and a professional attitude around the workplace will help to avoid any gossip and unwanted rumours. The workplace is for work, not for flirting, kissing or the like.
Honesty
Instead of being the office gossip, it is worth suggesting to the parties involved that they are best not lying or avoiding questions about their relationship if it has already leaked out to others. Lack of honesty is likely to encourage more office gossip and rumours and creating further distraction from day to day activities.
Is it real?
Ask the parties involved if their relationship is real and likely to last and if it is worth the upset that coming together could potentially cause if the relationship becomes public and other relationships are ruined.
Policy
Consider putting together your own workplace policy or manual surrounding office relationships and what is expected of your staff when it comes to relationships/affairs. This will help you to approach any issues that may arise in the future with new relationships.
Although you cannot control love and relationships of others, you can definitely help to manage and control how they are dealt with inside of the workplace and how they can impact others. Take each situation as it comes as they will all be different and most probably need to be handled with their own individual care and attention.