Mostly we recommend that you use MAME stand alone over the RetroArch version. Running MAME through LB is just like running MAME LB is just a launcher so if you want the interface that LB provides I would suggest using it. Usually if you map the controller and it removes the keyboard key just hitting enter again and pressing the key will bind it to both. MAME excepts button combos to when you set things up so if you press two buttons simultaneously it will map that as a combo as an example you can use start and select on your controller to act as the escape key and exit the emulator