Are you using a XMP Dimm? (this only works with one Dimm per channel). Generally, you should be able to put memory in either channel, without needing to populate the other channel for it to work. There is nothing in your motherboard manual about pre-populating channel A before Channel B. Of course you could verify this by putting a second piece of memory in.
No im not using XMP Dimm.
Spoke to the "technical department" apparently you have to pre-populate channel A before B on some of the new motherboards