This defies logic, unless the cable is so slack that you run out of lever travel with one side touching and before the other is forced to move.
Newton's 3rd law of motion dictates that the force then housing exerts on one arm is equal and opposite to that which the inner wire applies to the other. Likewise no arm can exert any force on the rim until the other comes into contact.
As I think about your problm, I suspect that you simply have the brake cable adjusted so long that you run out of trave,.
Try this, disregard the cable, and squeeze the upper and lower arms together (where the cable and housing are) and see what happens. If both move in your hand, they will when the cable is a attached.
Ohterwise (and don't get offended) two other possibilities exist.
1- you're trying to string a cable without housing
2- the housing coop is cut so short that it cannot move down pushing the brake arm before it.
Otherwise, a photo may be necessary for us to help.
__________________
FB
Chain-L site
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