No idea what you guys are doing wrong but I've been using dual PSU without an ADD2PSU since 2013 and never had issues.

Basically I just short the second PSU with a paper clip and start both at the same time.

Just keep them grounded and make sure no 12V mix between PSU and you will be fine.

You can interconnect the different PSU with different GPUs and risers. One PSU can power the GPU power and another the riser