Reusing the general approach from this post gives a way for setting Shift_L+Shift_R as layout change:
$ gsettings set org.gnome.desktop.wm.keybindings switch-input-source "['<Shift>Shift_L']"
$ gsettings set org.gnome.desktop.wm.keybindings switch-input-source-backward "['<Shift>Shift_R']"
This can not be done from the GNOME settings UI. It also works only accidentally, since it is not possible to be very intentional about left and right Shift modifier in the shortcut key definitions. Whew.