Need help with qmk

Hello everyone, I recently bought a piantor keyboard and I am having trouble with qmk. I have configured two layer keys(num, nav) each of them switches to different layer and I want to be able to hold both of them to access fourth layer(CzechLayer), but I can't seem to figure out how to do it. Here is my keymap.c file:

#include QMK_KEYBOARD_H
#include "keymap_czech.h"

enum layers {
    DefaultLayer,
    NumLayer,
    NavLayer,
    CzechLayer,
};

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    [DefaultLayer] = LAYOUT_split_3x6_3(
        KC_PIPE, KC_Q,         KC_W,         KC_E,         KC_R,         KC_T,          KC_Y,    KC_U,         KC_I,         KC_O,         KC_P,            KC_PPLS,
        KC_ESC,  GUI_T(KC_A),  ALT_T(KC_S),  CTL_T(KC_D),  LSFT_T(KC_F), KC_G,          KC_H,    RSFT_T(KC_J), CTL_T(KC_K),  ALT_T(KC_L),  GUI_T(KC_SCLN),  KC_PEQL,
        KC_UNDS, KC_Z,         KC_X,         KC_C,         KC_V,         KC_B,          KC_N,    KC_M,         KC_COMM,      KC_DOT,       KC_SLSH,         KC_PMNS,

                                             MO(NumLayer), KC_TAB, KC_SPC,                  KC_BSPC,  KC_ENT,  MO(NavLayer)
    ),

    [NumLayer] = LAYOUT_split_3x6_3(
        KC_QUOT, KC_1,         KC_2,         KC_3,         KC_4,         KC_5,          KC_6,    KC_7,         KC_8,         KC_9,         KC_0,            KC_PPLS,
        KC_ESC,  KC_EXLM,      KC_AT,        KC_HASH,      KC_DLR,       KC_PERC,       KC_CIRC, KC_AMPR,      KC_ASTR,      KC_BSLS,      KC_COLN,         KC_PEQL,
        KC_DQUO, KC_TILD,      KC_LABK,      KC_LCBR,      KC_LBRC,      KC_LPRN,       KC_RPRN, KC_RBRC,      KC_RCBR,      KC_RABK,      KC_QUES,         KC_PMNS,

                                             MO(NumLayer), KC_TAB, KC_SPC,                  KC_BSPC,  KC_ENT,  MO(NavLayer)
    ),

    [NavLayer] = LAYOUT_split_3x6_3(
        KC_BRIU, KC_F1,         KC_F2,        KC_F3,        KC_F4,        KC_F5,        KC_F6,   KC_F7,        KC_F8,        KC_F9,        KC_F10,          KC_VOLU,
        KC_ESC,  KC_F11,       _______,      _______,      _______,      _______,       KC_LEFT, KC_DOWN,      KC_UP,        KC_RIGHT,     KC_F12,          KC_MUTE,
        KC_BRID, _______,      _______,      _______,      _______,      QK_BOOT,       QK_MAKE, _______,      _______,      _______,      _______,         KC_VOLD,

                                             MO(NumLayer), KC_TAB, KC_SPC,                  KC_BSPC,  KC_ENT,  MO(NavLayer)
    ),

    [CzechLayer] = LAYOUT_split_3x6_3(
        KC_PIPE, _______,      CZ_EACU,      CZ_ECAR,      CZ_RCAR,      KC_T,          CZ_YACU, KC_U,         CZ_IACU,      KC_O,         _______,         KC_PMNS,
        KC_ESC,  CZ_AACU,      CZ_SCAR,      KC_D,         _______,      _______,       _______, _______,      _______,      _______,      KC_SCLN,         KC_PEQL,
        KC_UNDS, CZ_ZCAR,      _______,      CZ_CCAR,      _______,      _______,       KC_N,    _______,      KC_COMM,      KC_DOT,       KC_SLSH,         KC_PPLS,

                                             MO(NumLayer), KC_TAB, KC_SPC,                  KC_BSPC,  KC_ENT,  MO(NavLayer)
    ),
};