I got it working with the help of this comment and the esbuild ‘wrapper’ documentation here

What I did:

mix.exs file

  defp aliases do
    [
      ...
      "assets.deploy": ["esbuild bundle_one bundle_two --minify", "phx.digest"]
    ]
  end

dev.exs

  watchers: [
		# Start the esbuild watcher by calling Esbuild.install_and_run(:default, args)
    esbuild_bundle_one: {Esbuild, :install_and_run, [:bundle_one, ~w(--sourcemap=inline --watch)]},
    esbuild_bundle_two: {Esbuild, :install_and_run, [:bundle_two, ~w(--sourcemap=inline --watch)]}
  ]

config.exs

config :esbuild,
  version: "0.12.18",
  bundle_one: [
    args:
      ~w(
		js/bundle-one.js js/bundle-one.css
		--bundle --target=es2017
		--outdir=../priv/static/assets/bundle_one
		--external:/fonts/* --external:/images/*),
    cd: Path.expand("../assets", __DIR__),
    env: %{"NODE_PATH" => Path.expand("../deps", __DIR__)}
  ],
	bundle_two: [
    args:
      ~w(
		js/bundle-two.js js/bundle-two.css
		--bundle --target=es2017
		--outdir=../priv/static/assets/bundle_two
		--external:/fonts/* --external:/images/*),
    cd: Path.expand("../assets", __DIR__),
    env: %{"NODE_PATH" => Path.expand("../deps", __DIR__)}
  ]

Files names replaced for obvious reasons.
And it seems to be working.
Can anyone improve on this?