Ive been hunting around these forums all day and haven’t really come to a solution to my problem. I have seen similar posts, but they are not exactly like my issues. Sorry if i somehow missed a post.
So some info up front: Windows 10 build 18362 RetroArch 1.7.9 Controller #1: N30 Pro 2 (firmware 6.10) Controller #2: NES30 (firmware 4.20)
I have two (well actually more than that, but in this case I am trying just these two devices) 8bitdo controllers. One is a NES30, and the other is a N30 Pro 2. Both have their respective latest firmware (4.20 for the NES30 and 6.10 for the N30 Pro 2).
Outside of retroarch, each of these controllers appear to work fine, both individually and connected at the same time over bluetooth.
Individually each of these controllers works fine in retroarch over bluetooth. If i dare connect both over bluetooth for some 2 player action, the NES30 basically becomes inoperable. Some of its buttons work (a and b) but the rest of it stops. Interestingly enough, both controllers are seen by retroarch as “Xbox One Controllers” (and logs show it loading that config) when connecting them to bluetooth. This also causes the ports the controllers are assigned to also shuffle. My guess is that somehow retroarch doesn’t see that they are two separate devices, or at least that would make sense if one controller controlled both players in game, which doesn’t seem to be the case since one controller will work normally and only control one player, while the other will only work with its a and b buttons, and only control the other player.
If I connect the NES30 via usb, it loads the appropriate config, the popup in retroarch also reflects this. However, the controller doesnt work for the retroarch menus if both controllers are connected (N30 Pro over BT and NES30 on usb) In game, this seems to work fine though.
Of course the catch here is that I don’t really want to use these devices as wired controllers. I have a x360 controller for that 
I’ve set input_player1_joypad_index = “0” and input_player2_joypad_index = “1” Which doesnt seem to do anything to resolve the issue.
Ive tried to just custom bind them, but the device index only ever shows “XBox One Controller” (there are interestingly two entries if both devices are connected via BT, but only one of the entries seems to work.)
Ive also tried the nightly build of retroarch. In this case, retroarch properly figured out that these were indeed two controllers. Both connected over bluetooth and both registered menu buttons just fine, but when i loaded a game, the NES30 stoped working. Again, both controllers were also seen as “XBox One Controller”
What is also interesting is I see there are auto configs for these devices in the xinput folder. Both for their BT and USB modes. While the USB mode for the NES30 seems to work fine and load the right config, the N30 Pro 2 comes in as a generic xinput device, despite there being a config for it.
Just generally, all around, it seems that there are weird connectivity things happening and retroarch is getting confused about devices.