The answer is a double-pole, double-throw switch. I believe the momentary switch in the dna30 is of this type. It controls the two different circuits simultaneously.
You will need to draw out the button board traces on paper and confirm them with the continuity check function of the multimeter. The problem is that the shared B- will not work with the dna40.
You will need to draw out the button board traces on paper and confirm them with the continuity check function of the multimeter. The problem is that the shared B- will not work with the dna40.