Add 32G to R710 Server

I'm trying to upgrade the RAM in an R710 server (2 processors).

The original configuration of 32G is  8 x 4G DIMMS (Optimized)

DIMMS are in A1, A2, A4, A5 and B1, B2, B4, B5.

I have 2 x 16G DIMMS to add, and I cannot find a valid combination for how to add these in to make 64G.

I thought a channel could have just one DIMM, but it seems like that doesn't work.