many thanks Swizzy and SuRbO:
@Swizzy:
I'm not sure about a faulty rf-module ... so I add these other info's:
If I connect two wireless controllers, all it's OK (different channel for each, paring is good)
there is an user (I don't know if I can post link: it's a user's feedback about controller purchased on a famous web portal) with SAME problem ... he supposes that the problem is due to controllers that are PC compatibles
So, you say: XBOX kernel assigns channel to controllers, right ? ... and when it assigns the second one, doesn't remeber the first one ... unbelievable !
@SuRbO: it's a slim, RGH modded
a little upgrade: I connect, in this order, 1st wireless controller, it pairs 1st channel (upper left led); 2nd one wireless controller takes 2nd channel (upper right led), 1st wired controller pairs 3rd channel (lower left led) and, finally, 2nd wired controller pairs 3rd channel, so it excludes the previous brother !
many thanks, guys !