With two PSU's it is not that easy to turn them on simultainlasly.
With two PSU's it is entirely unnecessary to turn them on simultaneously. A screwdriver still works fine
When I "repower" our rigs, I only turn off/on the one that's connected to the MB and risers. The PSU powering the 6/8pin power plugs doesn't need to be powered off (unless it's powering risers, in which case just make sure you power it off second, and power it on first).