My cats were fine together for two years and now they fight constantly. Nothing changed.

Something changed. You may not have seen it, but something shifted in the environment, in one of the cats, or in the relationship between them, and it triggered a breakdown in the social structure they had built. Two years of getting along does not make cats immune to this. It actually makes the sudden change more alarming and harder to place because there is no obvious external event to point to.

The most common hidden triggers I look for in this situation are a medical change in one of the cats, particularly pain, which can make a normally tolerant cat reactive or alter how they smell to the other, an outside stressor like a neighborhood cat who has started appearing near windows, or a resource pressure that has been building slowly and finally tipped over. Two months is a meaningful window. 

Think about what was happening in the six weeks before the fights started, not just in your home but outside it as well.

Until you identify the cause, separating them and doing a structured reintroduction is the most important immediate step, because the longer the pattern of fighting continues, the more entrenched it becomes and the harder it is to reverse. The Work With Me assessment would be worth doing here because inter-cat aggression that develops suddenly in an established pair almost always has a specific trigger, and finding it changes everything about how you approach it.