Finally found the solution:
The default configuration of GPIONext is not optimal for those inputs.
What fixed it:
Hope it helps someone else !
@barbudreadmon thank you very much
@supercatfooz ahh ok sucks ah well can't want everything right heh
But it would be awesome tho if it worked
I remember playing a lot of Bomberman '94 on the PC Engine/TG16 with my girl friend back in the day! This version supported up to 5 players at once! I think there's been a Bomberman/Dyna Blaster game ported to just about every retro console, handheld and the arcade.