Left to the viewport controls there is the map moving section. Inside the dropdown you can find all imported scans and activate them by clicking on it.
After importing an additional scan to your project you need to position it correctly. Go ahead and click on “Move scans”. All windows disappear as you are in the “multiscan move mode” now. In the top left corner dropdown you can see which scan you are working on. It is also possible to move several scans at the same time. Keep in mind that active scans cannot be locked. Inside the bottom panel you can find position and rotation volumes.
Save your arrangement when you are done.