In a multiple PSU setup the reason you want to connect the risers to the the same PSU as the motherboard is because all devices connected to the motherboard MUST share the same power source and ground so the voltage can be regulated according to a single source.
https://bitcointalk.org/index.php?topic=1843586.msg18351224#msg18351224The problem with powering more than two risers per molex strand is 18 AWG wire is only rated for 10 A max for single core wire, which is 120 W at 12 V and much less for multi stand wire. Using more than two risers per PSU cable strand WILL exceed the rating of 18 AWG wire, causing the wire to heat up and is dangerous.
https://www.engineeringtoolbox.com/wire-gauges-d_419.html