In order for the mem to operate in dual channel mode, you must install a stick in each channel.
A1 + A2 = Channel #1
B1 + B2 = Channel #2
Therefore, you need to use one of the following configurations:
A1 + B1
A2 + B2
A1 + B2
A2 + B1
Go with A1 + B1.
-phil