For temporary session, this seemed to work for me with alt+shift toggle.

setxkbmap -layout us,gr -variant ,polytonic -option grp:alt_shift_toggle

To check current layout options you can run query and it would print currently active rules, model, layout, options.

setxkbmap -query

You probably dont need to do any reconfiguring if you find layout text file in

If you need to add or create layout, then you would need reconfiguring after changes with

dpkg-configure xkb-data

To check if there are errors in the file

setxkbmap -symbols /usr/share/X11/xkb/symbols/gr