Without cooperation of both buyers, you cannot do anything.
Both buyers should open NOT AS DESCRIBED CASES you provide return labels and on return refund in full.
Buyer A should be told to do this, Buyer B seems unwilling to cooperate.
The whole situation will be very expensive for you, and potential for bad feedback is high.
There is nothing that ebay can do, no one can force buyers to cooperate, but buyer A is likely to return at your expense for a refund, buyer B has got a bargain but no way to get item back.