AWS Availability Zone names may look like unique identifiers, but they are mapped to physical availability zones essentially at random. This means that us-west-2b in one account may be the same physical availability zone as us-west-2a in another account.
AWS VPCs make it possible to establish private network connections across AWS accounts with VPC Peering, essentially merging the networks into one. However, there is another option for cross-account/cross-VPC network access, with a much smaller surface area.