Creating an analytics report using 2 databases

I am trying to create a report that would combine similar data from 2 different databases. We have multiple businesses with the same data that run sales by month, but I’d like to show a combined sales amount.

Is this possible or of not can I close and modify one of the databases to include both sites?

To the best of my knowledge, the only way to merge data from two different sources/databases is two add them to the same database within the Designer.
It can be done as both a single stream (but ensure you have some property or dimension that allows you to distingiush the two sources from each other) or as two separete streams - your data and need should be considered. With the latter, a Custom Mode table/chart can be used to add nubmers from two separete streams together.