/usr/share/X11/xkb/symbols
Edit: /usr/share/X11/xkb/symbols/level2 (274B)
// These partial variants assign Shift to various XKB keycodes.
// The Less/Greater key (while pressed) chooses the second shift level.
partial modifier_keys
xkb_symbols "lsgt_switch" {
key
{
type[Group1]="ONE_LEVEL",
symbols[Group1] = [ Shift_L ]
};
};