Impossible to boot with more than 1 gpu

Hey guys !

I just built a rig with 6 rx 470 on a Asrock H81 btc 2.0 and i'm having an issue.

When I'm starting with 1 gpu at the position #0, there's no problems, i'm going to my OS (Ethos distro). BUT when I'm adding a 2nd GPU as #1, I keep going in the bios, I'm not able to access my OS (Well happened one or 2 times but when I restart it's falling again so its random)

Do someone got any ideas to help me ?

Thanks !!