I would strip both cubes, label one MB "A" the other "B" and the blades "A1,A2 .....B1, B2" etc
then move all A's blades to B and B's to A, try that if the fault moves to the other MB you have a faulty blade, if it not you have a faulty MB.
If it is a blade that is faulty, plug each blade in and test till you identify the faulty one. Then hit up your supplier.