Hello, is it yet possible to setup controller buttons (for example on NES) so they work like A+B. I mean map a single button on a controller that acts like A+B.
Furthermore is it possible to program one button. Lets say L1 which would act as if I have pressed B and then after 0.5 seconds button A. Imagine a NES game Turtles 3 when you do a special attack. But you could do that with a single button press.
You can not, unfortunately, RetroArch can not configure combos or sequences.
The only one that does it is the FBNeo core, and it handles several consoles, it is something native to the emulator, I don’t know if it is possible to configure custom buttons.
The first request, A+B, can be done by the FCEUmm core. The other one is not possible entirely, except by some external input mapping software, unrelated to RetroArch.
I have checked out FCEUmm core and nowhere in the input customisation did I see where and how can I set this up. Can you please be a bit more specific? 
After loading a game, go inside the “controls” submenu and you can map any button of your controller as the macro A+B.