map one keyboard key to two controller buttons

[nitrohepcat]

Looking for help on how to add a custom controller mapping in which one keyboard button equals a press on two buttons of the controller.
Specifically keyboard number 2 = right shoulder and left shoulder controller buttons. It can be a universal mapping for all emulators, or ideally it could be rom & emulator specific
Pi Model or other hardware: B+
Power Supply used: 2.5A
RetroPie Version Used: 4.7.1
Built From: Pre made SD Image on RetroPie website
USB Devices connected: usb keyboard
Emulator: lr-stella

[IanDaemon]

@nitrohepcat So...
Situation A) Do you want left shoulder to be "2" and right shoulder to be "2"?
OR
Situation B) Do you want left shoulder to be "half of 2" and right shoulder to be "half of 2"?

I'm not sure you map a controller input to "half a keyboard key".