As far as I can tell, based on experience only, it's a "first one wins" case, meaning that when one of the following happens, the review is completed and post is deleted:
- Three users with 20k rep choose "Delete".
- Six users with less than 20k rep choose "Recommend Deletion".
The order of the review actions doesn't matter.
Based on that, when a post has 3 "Recommend Deletion" and 2 "Delete" reviews, nothing happens. If a third user with 20k rep choose "Delete", it will be deleted, or if three more <20k rep users choose "Recommend Deletion".
Note: above applies only for answers with negative score. Answers with positive score can't be deleted from review, just send mod flag when review is completed.