Change GUI menu from one to the other

I've got an Inventory GUI that opens when hitting "E" and an Exit GUI that opens when hitting "Escape". I want one to close while the other opens when their related button is pressed. I've achieved this some what by adding GameObject.SetActive(false); to each button but I have to double tap each button. I press "Escape" once, it hides the Inventory, I then have to press it again to show the Exit GUI, and vise versa.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ActivateGUI : MonoBehaviour {
 
     public GameObject inventory;
     public GameObject exitMenu;
     private bool isShowing;
 
     void Update()
     {
         if (Input.GetKeyDown(KeyCode.E))
         {
             isShowing = !isShowing;
             inventory.SetActive(isShowing);
             exitMenu.SetActive(false);
         }
 
         if (Input.GetKeyDown(KeyCode.Escape))
         {
             isShowing = !isShowing;
             exitMenu.SetActive(isShowing);
             inventory.SetActive(false);
         }
     }
 }