MultipleReducers.js

const initUserState = {
    isAuth: false,
    isAuthModalOpen: false
}

const initGalleryState = {
    selectedPhotoURL: ""
}

const UserReducer = (state=initUserState,action) => {
  switch(action.type){
    case "SET_AUTH":
      return {
        ...state,
        isAuth: action.status
      }
    case "IS_AUTH_MODAL_OPEN":
      return {
        ...state,
        isAuthModalOpen: action.isOpen
      }
    default:
      return state;    
  }
}

const GalleryReducer = (state=initGalleryState,action) => {
  switch(action.type){
    // here goes your case statements
    default:
      return state;    
  }
}

rootReducer = combineReducers({user: UserReducer, gallery: GalleryReducer})
// the final state will look like below
{
  user: {
    // user state variables go here  
  },
  gallery: {
    // gallery state variables go here
  }
}

export default RootReducer