I thought for sure I fried something, because I thought both psu's needed to be started up at the same time. At some point I thought I read that it posed a threat to your board, if they were not powered on at the same time.
FYI, That splitter was
designed to power on the two PSU at the same time. The board will signal the power on signal.
Notice that there are only two wires connected in the other 24pin port, those are (green) PSON and (Black) Neutral wires if color coded.
If happened that the splitter was broken and the other PSU doesn't turn on, the MOBO will not be at risk, it will not POST and if you have a display (onboard or gpu connected to the turned on PSU), it would just stuck in the bios logo.