Follow-up from "Use streaming results instead of OFFSET/LIMIT on Geo backfilling workers"

The following discussion from !15457 (merged) should be addressed:

• @abrandl started a discussion: (+5 comments)

  @dbalexandre So we run two separate queries here but we continue the loop below until results from both queries have been retrieved? I'm unsure if we'd not just see results for the second query but haven't tested it. Do specs cover this?

  It looks a bit odd seeing the string concat here. WDYT about issuing separate send_query calls (and basically calling the loop below twice)? Is there a benefit from sending both queries at once?