You can open one of the Origin project, then expand Project Explorer, right click on the top level folder and select Append Project. Then navigate to the second OPJ and click OK button to append the project as folders into this current project.
You can new a project, in the Project Explorer , right click on the "UNTITLED" folder, in the context-menu choose "Append Project..." and select the project file you need to merge.
You can open one of the Origin project, then expand Project Explorer, right click on the top level folder and select Append Project. Then navigate to the second OPJ and click OK button to append the project as folders into this current project.