Joiner XYZ must have exactly two inputs.

Symptom

You get the error even if  your mapping joiner has exactly two inputs but at least one of them is a mapplet.

Reason (possible)

I do not know for what reason Informatica made this restriction.
I imagine the OUTPUT transformation to be sort of a virtual transformation, a placeholder as the mapplet transformation is. If you imagine the mapplet in the mapping being replaced by the content of the mapplet (like you can choose in the debugger) WITHOUT the OUTPUT transformation, you end up with more than two inputs for the joiner.

Solution

Create a collector expression as only input to the mapplet output.