Currently, it is not possible to merge two Azure DevOps projects into one board.
There is a feedback item Under Review to make it possible to move a Team Project between Team Project Collections.
You can explore the DevOps extension *Migration Tools for Azure DevOps, which allow you to migrate Teams, Work Items, and Plans & Suits from one Project to another in Azure DevOps/TFS both within the same Organization, and between Organizations. This Video Overview breaks down how to use it.
*Please be aware this is not an official Microsoft documentation/tutorial
Best,
Grace