Is it possible to have 2 sources for migration?
You can't use two sources in the same migration. But if you can split your import into two separate migrations (e.g. taxonomies first, then nodes), you can use either migrate_lookup (if your 2nd source provides the same primary key as a previous master) or entity_lookup (if your 2nd source needs to look up the referenced entity by something else than the primary migration key).

Entity lookup is part of Migrate Plus.