when combining the configs from pants.toml and pants.ci.toml, how are list combined? For example if I have different backends in

backend_packages

is a union of the two lists performed?

The pants.toml is read 1st, then the pants.ci.toml. Given that knowledge, stir in these rules and you should be able to get what you want - speak up if not: https://www.pantsbuild.org/docs/options#addremove-semantics