FYI, the documentation for setting up additional satellite services doesn't work; docker-compose complains about not being able to extend services that use 'depends_on'. Looks like it's a potentially known issue, but not sure anything is going to be done about it: services with 'depends_on' cannot be extended · Issue #3220 · docker/compose · GitHub
I was ultimately able to add the following to my docker-compose.override.yml to spin up a second satellite service; leaving here in case this helps others.
satellite-tradeideas:
image: 'quantrocket/satellite:2.5.0'
volumes:
- 'codeload:/codeload'
- 'db:/var/lib/quantrocket'
depends_on:
- codeload
restart: always