Is a follower's Witr prayer valid if their intention differs from the imam's, for example the imam performs three rakaat with one salam while the follower intended three rakaat with two salams?

Summary

By default, the number of rakaat may not be increased or reduced from what was intended at the start of the Witr prayer. If the imam intends three rakaat continuously and the follower intended two salams, the follower may intend separation at the final tashahhud of the second rakaat, then complete the last rakaah with the imam under a new intention. If the imam intends two salams and the follower intended continuous prayer, the follower follows until the imam's second salam and then completes the rest alone, like a latecomer (masbuq). If the follower is unsure how the imam will pray, it is permissible to make an unspecified (mutlaq) Witr intention without stating the number of rakaat.

Written in our own words from the original fatwa — check the source before acting on it.