A comma is normally placed before a coordinating conjunction (and, but, or, so, yet) that joins two independent subject-verb clauses—that is, clauses that could stand on their own as complete sentences. . . .
Parentheses can be used almost anywhere, but they are rarely seen in fictional dialogue or in quoted speech of any kind. The problem with parentheses in dialogue is that readers may not know exactly how to interpret them. . . .
Most writers and editors learn not to join (or splice) two independent clauses with a comma alone. . . .