You cannot merge two Jira Cloud instances into one using only site backup files.

Jira Cloud site backups are designed for single site restore workflows and a site restore replaces the target site rather than merging with it. Restoring a backup will recreate the backed up site's data and configuration and it will not combine two separate backup sets into one unified site.

To combine content from two cloud sites you must use targeted migration methods or work with Atlassian support to plan a merge. Common approaches include selective exports and imports or vendor supported cloud to cloud migrations, but those are not achieved by applying multiple site backup files to a single instance.

Create site backups for both instances and restore them into a new Cloud instance is incorrect because you cannot apply two different site backup files into the same target site. Restoring one backup will define the site state and a second restore will overwrite that state rather than merge it.

Create a backup of one instance and restore it over the other instance is incorrect because restoring one backup over another would replace the destination site and destroy the original data on that site instead of combining content.

Use the Jira Cloud Migration Assistant or perform a cloud to cloud migration is incorrect in this question context because the Jira Cloud Migration Assistant is primarily intended for Server or Data Center to Cloud migrations and there is no simple self service tool that automatically merges two independent cloud sites. Cloud to cloud moves often require planning and support from Atlassian.

Export issues from one instance and import them into the other using CSV is incorrect as a complete solution because CSV import only moves selected issue data and it will not preserve many elements such as full history, attachments, user mappings, permissions and some configuration details. CSV can help migrate issues but it does not perform a full site merge.

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