I'd like to be able to merge (link) facets without having to do so in code. For example, title, category and tags for autocomplete. You could have 'link' groups in the admin backend and if you assign a facet to a group it automatically does the remapping.
Sorry, but we have no intention of supporting multiple sources per facet.
It would add unnecessary bloat to the core + UI (majority of users will have no need for this), plus I think our existing tutorial is sufficient for this.