How can we create a single report of two unrelated objects using report api?
Salesforce doesn't allow to combine two unrelated objects either from UI or through API for reporting purpose.
Even if in Joined report works on the relations.
A joined report can contain data from multiple standard or custom report types. You can add report types to a joined report if they have relationships with the same object or objects. For example, if you have a joined report that contains the Opportunities report type, you can add the Cases report type as well because both have a relationship with the Accounts object
You can create a single report freshly and try to implement those logic (filter or field display or grouping) into new report.