There is no “SonarQube merge” feature as such. Then your 2 options are:
Use the the Project Move feature (for Enterprise Edition only) in order to export from one instance to import on the other. Please read carefully the linked documentation as there are some prerequisites. Please also note that this feature does not support branches, only the main branch will be exported/imported.
If the project/issue history is not that important to keep, you should simply start analyzing your projects on the targeted SonarQube instance. It’s also usually a good time for a cleanup.