I have connected both. Maybe one would be enough, but with both you are on the safe side.

The manual says for ATX_12V_1 and ATX_12V_2: Ensure to connect the 8-pin power plug, or connect both 8-pin power plugs.

For me, this means that it should also work with one. Depending on the power consumption of the CPU, I would definitely use both, because the ATX 8-pin cables are only designed for 150W according to the standard.