Apply thermal paste to both CPUs. The past is applied horizontally in a line and pea fashion. Do not put more paste than needed on the chips.
When re-assembling the CPU heatsinks, tighten the securing bolts according the to the X pattern that you used when taking them off. Tighten only finger tight.
After the heatsinks have been reattached to the CPU PCB, put the CPU Tray out of the way, on the edge of the lid.