It would be a 32GB dimm in A1 and B1,  and then a 16Gb dimm in A2 and B2.
Can I mix 2 32GB DELL A8711888  With 2 16GB DELL A8711887, DELL SNPHNDJ7C/16G 32GB RDIMM, 2400MT/s, Dual Rank, x4 Data Width (Chips Organization X4) 16GB RDIMM, 2400MT/s, Dual Rank, x8 Data Width (Chips Organization X8) Thanks  
Mixing 32GB and 16 GB memory on the T630