@zehanort You cannot set hotkey & Fast forward on the same button, the most you can do is set the hotkey to a button other than select (not recommended, but possible), and set fast forward to another button.
Hotkey = R2
Fast forward = L2

You can make a Configuration overload file for this, for an entire console, or, just for a game, read the documentation:

https://recalbox.gitbook.io/documentation/advanced-user/configuration/configuration-overload https://recalbox.gitbook.io/documentation/advanced-user/configuration/configuration-overload/retroarch-overloads

Use the survey tool and you will notice that some users here on the forum have already successfully done configuration overload (not for this purpose).