Hi Ben,
Unfortunately, currently you cannot link items in one project to items in another project due to application handling the security by Projects. We already have a corresponding feature request in our backlog to allow cross-project traceability linking. I've linked your request to this task to increase its rate.
Also, you can give kudos to the existing improvement idea in our Community:
https://community.smartbear.com/t5/QAComplete-Feature-Requests/Allow-linking-to-an-item-in-another-project/idi-p/101416
Anastasia
Customer Care Team