Finally it works! I followed the advice of disconnecting sata/molex that was used to power my HDD and the add2psu, and instead running a direct molex from the primary psu to the add2psu. 2 GPUs and USB Powered risers on the secondary GPU.  Also on the 2 GPUs powered by the secondary PSU, I replaced the 6pin (2x3) molex to 8 pin cables, with PCIe cables. With all of this the secondary PSU switches on!
Thank you.
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You're welcome.
I'm always glad to help to people, when I see they put some effort to solve the problem by itself.
It doesn't matter is successfully, or not. Main thing is, they at least tried to do something about it.
So, enjoy your hash