@ToastyGuRu
From what the ASRock manual says, and my own mobo as well. If you have one stick, place it in A2. if you have 2 sticks (your question) put it in A2 and B2.
in your motherboard, A2 is the second one from the CPU. so leave the closest slot to cpu empty. B2 is the furthest away from cpu. (page 13 of the pdf)
all those charts from page 27 on : doesn't matter which cpu or ram frequency or rank you have, it always go to A2 and B2. no wonder 93 pages for just one language!
my motherboard's manual is 186 pages covering 5 languages, english ends on page 46.
my manual says : always insert memory modules in the DIMMA2 slot first.
ASRock says the same thing with more jibber jabber.