Get a chunk of spec files. Use it like 'cypress run --spec $(node scripts/cypress-spec-chunk.js 4 0)'.

cypress-spec-chunk.js

#!/usr/bin/env node

/* tslint:disable:no-console */

const find = require("find");
const path = require("path");

const projectDirectory = path.resolve(__dirname, ".."); // because it is in ./scripts

const printChunk = (testFiles, totalChunks, chunkIndex) => {
  const chunkSize = Math.ceil(testFiles.length/totalChunks);
  const chunk =  testFiles.slice(chunkIndex * chunkSize, chunkIndex * chunkSize + chunkSize);
  console.log(chunk.join(','));
};

const main = (dir) => {
  const totalChunks = process.argv[2];
  const chunkIndex = process.argv[3];

  const files = find.fileSync(/\.spec.ts$/, `${dir}/cypress/integration`).sort();

  printChunk(files, totalChunks, chunkIndex);
};

main(projectDirectory);

package.json

{
  "scripts": {
     "cy:parallel": "concurrently -n 'cy1,cy2' -p '{time}|{name}' 'cypress run --spec $(node scripts/cypress-spec-chunk.js 2 0)' 'cypress run --spec $(node scripts/cypress-spec-chunk.js 2 1)'" 
  },
  "dev-dependencies": {
    "find": "xxx",
    "concurrently": "xxx"
  }
}