Merging 2 Power BIs

As lbendlin indicated, you cannot just drag/drop data. However, if the reports have different underlying data, you can do this:

1. Open file one. Open Power Query via Transform on the home ribbon.
2. Open file two. Open Power Query.
3. In file two, copy ALL queries in the Power Query window. Select all of them and press CTRL-C.
4. In file one, press CTRL-V.
5. Close and Apply in File one.
6. In file two, just close.
7. In file two, copy all visuals by selecting them, CTRL-C, then paste into file one.

The only thing you will have to do now is:

1. create relationships in the Model tab.
2. Create measures and calculated columns if any. You copy the DAX code for this but not the measures/columns themselves.

If the data sources in file 1 and 2 are similar or the same, but with different transformations, it can be a bit trickier as you will duplicate the sources and possibly have naming conflicts on the queries.

WIth the December 2020 desktop update, you can also enable the preview feature for direct query of datasets. You could then create a third PBIX file that brought in the other two datasets and create visuals and relationships as desired. But that is in preview status, and would require 3 final reports vs consolating into 1. But if your first two reports are very complex with a lot of DAX, that might be faster.