This is a pretty unusual design for a CLI. parseArgs is intended as a basic utility, not something which supports every possible CLI design; I don't think it's worth supporting this one.

You can do this with the tokens array as follows, though:

import { parseArgs } from 'node:util';

let { tokens } = parseArgs({
  options: {
    'files-to-read': { type: 'boolean' },
    'files-to-write': { type: 'boolean' },
  },
  allowPositionals: true,
  tokens: true,
});

let read = [];
let write = [];

let target = null;
for (let token of tokens) {
  if (token.kind === 'option') {
    target = token.name === 'files-to-read' ? read : write;
  } else if (target === null || token.kind === 'option-terminator') {
    throw new Error('names must be specified after --files-to-read or --files-to-write');
  } else {
    target.push(token.value);
  }
}

console.log({ read, write });

$ node parseargs-pos.mjs --files-to-read file1.txt file2.txt --files-to-write file3.txt file4.txt
{
  read: [ 'file1.txt', 'file2.txt' ],
  write: [ 'file3.txt', 'file4.txt' ]
}

I don't mind invest time in order to add this option.
The question is whether it's something worth adding to the codebase (in your opinion)

My 2 cents: as a user I wouldn't expect the --files-to-read file1.txt file2.txt format tbh.

I'd rather expect some other more common ways like comma-separated or space-separated lists:

# comma-separated list
$ node arseargs-pos.mjs --files-to-read file1.txt,file2.txt

# space-separated list, but explicitly passed as a single string
$ node arseargs-pos.mjs --files-to-read "file1.txt file2.txt"

Which are already feasible with the current features.

Having the option you propose would be confusing to me. I hardly believe it would be widely used, and it would probably end up being a uselessly maintained feature in the long run.