zmk/app/tests/mod-morph/behavior_keymap.dtsi

/ {
    behaviors {
        mod_morph: mod_morph {
            compatible = "zmk,behavior-mod-morph";
            #binding-cells = <0>;
            bindings = <&kp A>, <&kp B>;
            mods = <(MOD_LSFT|MOD_RSFT)>;
        };
    };

    keymap {
        compatible = "zmk,keymap";

        default_layer {
            bindings = <
                &kp LEFT_ALT &mod_morph
                &kp LEFT_SHIFT &kp RIGHT_SHIFT
            >;
        };
    };
};