I do not think it is directly supported, but you can use the command line setxkbmap with the generic custom shortcut keys. 
You have first of all to find the exact command that sets you keyboards; in my case I have 
 setxkbmap es -variant standard_tlde 
for the my custom Spanish keyboard and 
 setxkbmap us -variant altgr-intl
when I want the US-AltGr international one. The exact name of the variant is found in /usr/share/X11/xkb/symbols/es (or us, or it --- change as needed); for example, for the US variant I use, which is called "English (international AltGr dead keys)", it's here: 

When you have it, open Settings -> Keyboard -> Shortcut Tab, go on Custom Shortcut, and add (with the +) the shortcuts:

Type a name and the command you need. After that, you need to click on the "Disabled" word and type your shortcut:

And you're set. 
Notice though that keyboard layout switch is plagued by several and important bugs, so you will have to use a lot of patience...