How to run ram in dual channel?

Interestingly enough, the manual recommends you install modules in A2 and B2. But will also run fine in dual channel in A1 and B1.

I'd go with the manual recommendation though whether it makes a difference or not. There is some reason, whether for uniformity on support calls or for some other reason they would recommend this otherwise there'd be no reason why they'd publish this in the manual.