I’m not exactly sure what you mean by “linking forms”. Let me try to clarify some standard terminology that CommCare uses.
Forms can refer to two different parts of CommCare: the form definition (i.e. the structure of the form that is sent over to mobile users to fill in) and form data (a record of all user-generated data that is created during a mobile user’s form submission). I’m not sure whether you want to link form definitions or form data.
In either case, this is not possible across projects, or “project spaces,” as we call them. At a high level, a CommCare project space (for example, https://www.commcarehq.org/a/myprojectspace) is a container for all applications and project data (form and case data, look-up tables, locations, etc). Project data can be “linked” across multiple applications within the same project space. Entire applications can even be copied from one project space to another. But in two different project spaces, the form data and structure will be entirely separate, and unable to be linked together.
I hope this is helpful. Please feel free to add additional details if needed.