Split a sentence only when each half can stand alone, or when one half is a legal dependent clause with a period behind it. Joining the other way is in sentence combining.
Two complete thoughts, or not
If the second half starts with which or a participle, a period creates a fragment. A comma splice is the opposite error, in comma splices.
Adding a new sentence at a caret is a different question, in adding a sentence here. Passage order is in macro logic.
The bottom line
Split only if both sides are legal sentences. Fragments are not a fix.
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