yeah I figured
left stick + abxy + lr for player 1, c stick + d-pad + zlzr for player 2
that way player 1 still has normal controls but there's room for player 2 if you use input redirection and hook up controllers
wonder why you can't just map player 2 buttons on to player 1's device since 3ds only has one device- the 3ds.
man I just really want two player fighting games and whatnot considering the FANTASTIC library of them whether it be console, arcade or even handheld
pity the 3ds emulation scene is like 95% dead because of the Switch and the only emulator being really worked on with significant stuff being added is the very-in-progress N64 emulator
I just checked this and I was able to play Super Street Fighter II on genesis plus gx with two players. I realise your settings are a bit better than the cumbersome ones I thought of:
player 1:cpad=dpad, dpad=abxy, l1,l2,=l,r
player 2:cstick=dpad,abxy=abxy, r1,r2=l,r
I didn't manage to get it to work with picodrive though but that might be an error on my side. Yep it was an error on my side and works if you follow the steps below. I'm curious to see if it works with all retroarch emulators now...
Now if there's input redirection done through android we could enjoy two player anywhere. Found that as well. Haven't tested it out though:
https://github.com/JambonBeurreMan/InputRedirectionClient-Qt-Android
Also, if you're even more crazy you can connect a second 3DS as a controller to your android phone by using this (NB I haven't tested this

):
https://github.com/hacker1024/ctroller-android-app
https://github.com/hacker1024/ctroller-android
It's of course also possible to connect a 3DS to a PC like a controller.
The simplest way of course is just to use a normal controller connected to a PC, or even an android phone. You can do this by using this program:
https://github.com/TuxSH/InputRedirectionClient-Qt/releases
I've tried several apps to remap my wii u controller connected through magic-ns adapter but none of them work (:x360ce, Durazno, XInputPlus). The only remapping I was able to do was with WiinUSoft having my wii u pro controller connected to my pc with bluetooth.)
Input redirection on the 3DS is done through rosalina menu accessed by pressing L+Dpad down+select. Then go to miscellaneous options and start inputredirection. Take note of the ip address on the top right and use that in whatever input redirection client you have.
EDIT: If you haven't figured out how to do it then follow this: Go to "settings/input/user 2 binds" and set user 2 device index to 3ds controller. Then run your rom and go to controls in the quick menu. From there go down to the second set of auto: Circle Pad Right inputs and when it says User #2 in the bottom left corner. Set the inputs you want for player two on the Circle Pad and Cstick. Also make sure player two has a device type that actually works with the game on the top of the controls menu. This has worked with every emulator that supports control remapping in retroarch.
Cores I've tested so far:
FCEumm - Works.
Genesis Plus GX - Works.
Mednafen PCE Fast - Does not work - You can't remap controls.
PCSX Rearmed - Works (!)
Picodrive - Works.
Snes9x 2002 - Doens't work - Doesn't let me change controls in the quick menu.
Snes9x 2005 - Works
until I redirect input and it just refuses to play past the first few seconds of the game. It's happening even without redirecting now...
Snes9x 2005+ - Works.
Snes9x 2010 - Works.
Now my only complaint is that Retroarch for some reason doesn't allow you to use the abxy, shoulder buttons or start and select as inputs for player 2 for some reason. Maybe we should request this to be fixed since it seems very trivial but so necessary to get enough inputs to work with.
Also I would say the 3DS scene got pretty revitalized by the n64 emulator. Personally I enjoy seeing how far gbarunner2 has come. Heck it has even surpassed mgba and higan in accuracy on some (albeit) few roms! (Pacman World 1+2, Grim Adventures and possibly more).