As per the Bitbucket AWS Documentation and the supported Cloud Formation Templates, the recommended setup is to have active-active nodes across availability zones.
Please see the documentation at https://aws.amazon.com/quickstart/architecture/bitbucket/
The diagram suggests having active nodes across Availability Zone 1 and Availability Zone 2.
However, Atlassian support team does not recommend this configuration. As per support, one of the availability zones is meant to be a cold stand-by.
As a customer, I would expect the product to work as per the published architecture diagram to ensure high-availability across availability zones.
Please watch this issue regarding the documentation fix pending for the bug