Create Package Diagram with two packages named PackageA and PackageB.
Associate new diagram to PackageA – click on Add New Diagram in context menu of package (context menu available on right click).
Select Class Diagram, type its name and confirm it clicking on OK button.
Draw a class diagram.
Repeat steps 2 to 4 for PackageB.
In Project tab of Side bar, choose All Entities view.
Open the package diagram with PackageA and PackageB . Drag ClassA from project tree and drop it to PackageA and ClassB to PackageB.
Insert relationship between two classes. For example choose Use relationship and drag from ClassA to ClassB.