Good evening, I don't believe one replica is used primary and one for secondary. A write is not acknowledged until it is written to both replicas. As for which one a read would come from, not sure. Some fancy math algorithm figures it out.
Don't forget this all changes in v6.2 with RAID 5/6. VSAN 6.2 Part 2 - RAID-5 and RAID-6 configurations - CormacHogan.com
Thank you, Zach.