By default, customised Illustrator shortcuts are saved as kys files to the preferences folder in following locations on Mac and Windows:
Mac: user/Library/Preferences/Illustrator CC
settings/[language]/mycustomshortcut.kys
Windows: Users\\[user name]\\AppData\\Roaming\\Adobe\\Adobe Illustrator CC Settings\\[language]\\mycustomshortcut.kys
The files can be copied to the same location on another machine and you will be able to choose the set of shortcuts in the Keyboard Shortcuts dialogue in Illustrator. You should be able to use the same .kys file across platforms as long as the modified shortcuts are valid.