There are 2 ways to add another report tab to the report you are working on.

The first is to use the + button in the top right corner of the report window.

While working in the current report tab, select the arrow drop down of the report tab and select "+New"
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Now there will be two tabs, Report 1 and Report 2. Click on a tab down arrow to select rename it to name it something more useful.
​Please note that Report 2 is blank by default.
Click the Templates tab on the left and drag and drop a Table onto the report. Then, click on the Data tab on the left and drag and drop fields as needed.