dual channel memory

My board supports dual channel memory.
I purchased the memory that is tested in pairs, especially for dual channel boards.
My question is this:
There are two banks, A & B
Two memory chips in the pair.
Does one of the pair go in A and the other in the coresponding slot in B, or do both pieces from the pair go in the A slots? (or B slots)
There are 2 slots in the A bank and 2 Slots in the B bank.