$ npm install koa-mount

const mount = require('koa-mount');
const Koa = require('koa');
// hello
const a = new Koa();
a.use(async function (ctx, next){
  await next();
  ctx.body = 'Hello';
});
// world
const b = new Koa();
b.use(async function (ctx, next){
  await next();
  ctx.body = 'World';
});
// app
const app = new Koa();
app.use(mount('/hello', a));
app.use(mount('/world', b));
app.listen(3000);
console.log('listening on port 3000');

const mount = require('koa-mount');
const Koa = require('koa');
async function hello(ctx, next){
  await next();
  ctx.body = 'Hello';
}
async function world(ctx, next){
  await next();
  ctx.body = 'World';
}
const app = new Koa();
app.use(mount('/hello', hello));
app.use(mount('/world', world));
app.listen(3000);
console.log('listening on port 3000');

app.use(mount(a));
app.use(mount(b));