I want to combine 2 android projects together in such a way that when an button is pressed from one activity (of 1st project ) it should start my activity from (2nd project)
The answer depends on what you intend to do with the projects.
If you wish to merge the project into a single app , at least one of them should be set as an android library , and another project should use it . This way , the merged projects would be able to recognize each other .
If you wish to have 2 apps and not a single app , each activity that you wish to access from another activity should have a special definition in the manifest (using intent filter) of how to open it , as it's not the default behavior .