ping.mjs

import { spawn } from 'child_process'

const cp = spawn('node', ['pong.mjs'], { shell: true })

cp.stdout.pipe(process.stdout)
cp.stderr.pipe(process.stderr)
process.stdin.pipe(cp.stdin)

pong.mjs

process.stdin.on('data', (data) => {
  process.stdout.write(data)
})