CombineReducersのインポート例

rootReducerにreducerを注ぎ込む。
このrootReducerの部分はuseSelectorで取得することができるstateになる。

index.ts

import { combineReducers } from 'redux'
import {
  FirstReducer,
  SecondReducer,
} from './reducer'

const rootReducer = combineReducers({
  first: FirstReducer,
  second: SecondReducer,
})

export default rootReducer

各reducerの例

actionのpayloadは対象のreducerが担当するstateの分だけ用意すれば良い。
CombineReducersがそれらを結合してくれる。

reducer.ts

const initialFirst = {
  hoge: '',
  foo: '',
}
const initialSecond = {
  bar: '',
  baz: '',
}

export const FirstReducer = (
  state: typeof initialFirst,
  action: {
    type: string,
    payload: typeof initialFirst
  }
): typeof initialFirst => {
  switch(action.type) {
    case 'HOGE':
      return {
        ...action.payload,
        hoge: action.payload,
      }
    case 'FOO':
      return {
        ...action.payload,
        foo: action.payload,
      }
    default:
      return state
  }
}

export const SecondReducer = (
  state: typeof initialSecond,
  action: {
    type: string,
    payload: typeof initialSecond
  }
): typeof initialSecond => {
  switch(action.type) {
    case 'BAR':
      return {
        ...action.payload,
        bar: action.payload,
      }
    case 'BAZ':
      return {
        ...action.payload,
        baz: action.payload,
      }
    default:
      return state
  }
}

combineReducers · Reduxも参考に。