This is only possible in a 1 CPU configuration (2nd CPU heatsink space will be ocupied by add-on cards)
you can use the following risercards:
For left side:
RSC-R2UU-UA3E+ / RSC-R2UU-UAX (rev. 2.0) / RSC-2UU-A3XE8 (rev. 2.0) / RSC-R2UU-A4E8+
For right side:
RSC-R2UU-2E4R