merging of activity results from different projects is not trivial task.
so if you are planning to merge activity results it's better to work in one project adding different activities there.
yet there is one trick that enables you to merge activity results from different projects.
Open project "1" and "Pack&Go" it.
Open Project "2"
Unpack the file with packed project "1" inside the open project "2". (Use the check box "unpack to the current project")
Now you have all activity results in one project. You can merge any.