If your business has a separate policy for Harassment or Bullying follow this.
Otherwise, follow your normal grievance procedure.
You will follow the first two stages of the AIMLA model:
1) Acknowledge the complaint
2) Investigate the complaint, gather evidence - which will include speaking to the person that is the alleged perpetrator of the complaint
If the allegations against the perpetrator are serious you may need to consider suspending that employee.
Remember this is not a punishment, you may feel the investigation or the wellbeing of either employee may be compromised by keeping them at work.
This is a potentially complex situation so we recommend you to take professional advice.