Terminology clarification first. Let's say, plurality herein means the quality of being plural; the state of being plural or not, which is not the dictionary definition. I couldn't find another appropriate term for it. Well, grammatical numerity? Hmm.
In this sentence, the number (plurality) of the subject changes at the middle of the sentence. The subject was singular, but soon becomes plural.
It's split into two and merges/merge back into one a hundred times.
We can also think of an inverted version of it. The subject was plural, but soon becomes singular.
The two are merged into one and (are)/is split back into two a hundred times.
How do I deal with this kind of plurality? What should I assume its plurality is? Do I have to break it down into two or more sentences?