2 memory chips with 4 slots?

The manual isn't the clearest directions I've seen before but what I gather is putting both in either A_1 & A_2 or B_1 & B_2 is your best option over A_1 & B_1 or A_2 & B_2. This format/method has the most support for the RAM setup based on the QVL.

Fill A1 and B1 for dual channel operation, if your motherboard is capable of dual-channel. If you fill A1 and A2, you have single channel operation, which is not good provided again that your motherboard supports dual-channel. B1 and B2 also operate in single channel only.

Put them in the matching colored slots.

For only two RAM sticks, the use of the term "matching colored slots" may not be a good idea as the remaining slots ( A2 and B2 ) are also matching colored slots. But they may not be detected by the motherboard unless you fill up the A1 and B1 first.

In my motherboard ( P7P55D ), the slot closest to the CPU is A2 ( colored black ) and often the slot preferred by the amateur builders because of its closeness to the CPU together with the other black slot. Then they come up and say " our RAM sticks are not detected ".