Writing a custom migrate process plugin for multiple entity_lookups matching the same entity

Question

I'm working on a migration of Album nodes, and before importing a given row, I'd like to do an entity_lookup that checks if there exists an Album node that has both a matching title and artist (field_artists), skipping the row if there is a match.

My understanding is that I'll need a custom process plugin to handle that, but I can't wrap my head around how to write it so that it does the multiple entity_lookups properly.

This is the initial YAML from when I was only doing an entity_lookup against the title (which works, but isn't sufficient, given that two albums by different artists could very well have the same title):

process:
  skip:
    -
      plugin: entity_lookup
      entity_type: node
      bundle: album
      bundle_key: type
      value_key: title
      source: name
      ignore_case: 1
      access_check: 0

Here's my YAML in its current state, in which I'm not sure how best to structure sources:

process:
  skip:
    -
      plugin: entity_lookup_multiple
      entity_type: node
      bundle: album
      bundle_key: type
      value_key: title
      sources:
        name:
          value: title
        field_artists:
          value: field_artist
      ignore_case: 1
      access_check: 0
      method: row
      message: 'Album already exists; skipping row.'
  [...etc.]

The plugin, EntityLookupMultiple.php, is where I get lost: I'm not sure whether to extend ProcessPluginBase like it appears other migrations do, or if I need to extend EntityLookup directly.

class EntityLookupMultiple extends ProcessPluginBase { ... }

Then within that, I see that I think I can dig into my sources array using $this->configuration['sources'], and I see that I should throw a MigrateSkipRowException if both lookups match. In between that, how do I perform an entity_lookup against both values specified in the row (title and artist) and see if there's a matching entity that satisfies both requirements?