Well you should probably start by defining each stage in their relationships.
1. How and Why did they become friends in the first place? How long where they friends? What were they typically like when they were?
2. Why did they become enemies? How? What happened to break them apart? How long were they enemies? How did they act towards eachother this way?
3. Why did they go back to being friends? What lead to the decision to reunite? Has the past been forgiven?
4. When did they become lovers? Why and how? How long have have they had romantic feelings for eachother? Since the begining? Only recently? What affect did their love have on their previous stages?
Once you’ve broken down all the stages of their relationships, keep that informtaiton in mind as you write them. No matter what stage the story currently takes place in, all you have to do keep in mind what they have been through together. That information will seep into your story.
Also keep in mind that all the reasons should be major. Their relationship wont seem very realist if the reason why they seperated in the first place isnt beliveable, and it their relationship will be even more unrealistic if you dont give them an even better reason for making up and letting go of the past. Make sure to treat each stage fairly, spend an equal amount of time devolping each one. They have to be balanced in order to read correctly and believable.
I hope I was able to help you, and if you have any more questions, or something wasnt clear, feel free to contact me!