Show their growth throughout the fanfic. If they start off bitter towards each other, let them gradually change as they face different situations together. This development will keep the readers engaged as they see the characters evolving beyond their initial 'ex' status.
Give them distinct personalities. For example, one ex could be very outgoing and the other more reserved. Their differences can lead to interesting interactions.
Create backstories that explain why they became exes in the first place. Maybe one had a fear of commitment due to a bad family experience. These backstories will make the characters more relatable and add depth to their current relationship in the fanfic.