Last tested: Oct 23, 2018
Yes! All data in BigQuery is universally addressable. The full address of any table in BQ is projectname.datasetname.tablename and queries can join tables from different datasets and even different projects without a problem.
However, this can only happen if the account running the query has permission to query both tables. Since Looker generally connects to BQ via a service account, you need to confirm that the service account in Looker's connection has access to all the data you're trying to access. If it doesn't, you'll get a permission denied error.
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