Step By Step
Select + X to get into RGUI.
RGUI Quick Menu, Options, Display Mame Menu. When you resume from Quick Menu. Mame Menu will be launched.
The only downside from launching Mame Menu from RGUI is that the only way to get rid of it is to go back into RGUI and turn it off.
When Mame Menu is launched we see input (general) and input (this game)
We want input (general) so we set these hot keys for all Librettro mame 2003 games.
select+L2 for P2 Start.
select+R2 for Mame Menu launch (so we don't have to go through RGUI)
These Instructions work great for Mame 2003 Librettro based games.
But I can't seem to find a mame menu for fbNeo core.
There is none. Mame only.
https://retropie.org.uk/forum/topic/27618/player-2-start-with-1-controller
https://sites.google.com/view/gregsqna2/gaming/arcade1up/centipede/emuctrls/mamemenu