Some more progress...
1- Huzzah for rolling updates ! I updated the box before following the steps you proposed, @Substring.
2- I did the following: delete 99-8bitdo,rules, update box, forget controllers, reboot.
When pairing first device ( @OyyoDams : SFC30), I could pair and configure via the GUI. I can go through the ES menus with the pad and start games, but the pad is not recognized by the emulator (at least the SNES one says something along the lines of 'SFC30 gamepad not configured'. For the record, the emulator also says that my X-Arcade is not configured but I can use it properly so maybe this is a false lead.
When trying to pair my second device (SNES30), I saw it in the pair list but when selecting it, it said 'unable to pair'. However, after a reboot the pad was instantly paired when switched on. I could use it to go through the menus, I could configure it via the GUI, but as for the other one, it is not recognized (at least by the SNES emulator, maybe I should test other emulators and see if it's the same there [EDIT] Same thing for other emulators (GameBoy, Genesis) [/EDIT]).
Bottomline: the pads are paired and configured, can be used for menu navigation but not for playing games.
Hope this helps, looking forward to any further tips. My .rules files now has 3 times the SFC pad and 2 times the SNES pad...