Please explain multiple images in an executor

I can’t wrap my head around the concept of having multiple images in the same executor. Is there somewhere in the docs that explains what functionality this enables / doesn’t enable?

Or to put it another way, what’s the point of it?

From reading this blog post, I understand it’s possible for one container to communicate with another by exposing a port + executing REST api requests etc, but it doesn’t seem to suggest doing that in the docs.

I’d like to be able to access the binaries in one (secondary) container from the primary one; is that possible?

e.g. at the moment I am building a Wordpress theme which compiles some CSS & JS. I’m using the CircleCI PHP image, and I’d like to install packages with a certain version of node.

So, I thought it might be possible using the following syntax:

    executors:
      docker-php:
        docker:
          - image: circleci/php:7.3.0
          - image: circleci/node:6.17.1

The idea here would be to clone the repo to the PHP image, and then build CSS/JS using the node binary on the node image - is this possible somehow?

Thanks