Why do you want to join them?
If A & B have no connection to C,D, and E then A & B are probably better with a sentence of their own.
If C is connected to both D & E you may be able to join them; the manner would depend on the relationship.
Expanding on the subject a bit, the decision to join sentences or not has more factors than simply having a connection. You also need to consider clarity, length, flow, balance, context, and the previous and subsequent sentences.
In short, just because a pair of sentences can be connected doesn't always mean they should be.
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