Can I link two tickets that are present in different workspaces?

Yes, two tickets that are present across workspaces can be linked to each other. 

Incidents: Incidents can only have incidents as child tickets. Therefore, when you add a child ticket to an incident, you will only be able to see IT Workspaces that have “Incident” as a ticket type enabled.

Service Requests: Service Requests can have Service Requests/Cases/Queries/Issues/Requests as child tickets. Therefore, you can add any ticket from business workspaces and service requests from IT workspaces as child tickets.

Cases/Queries/Issues/Requests: These can have Service Requests/Cases/Queries/Issues/Requests as child tickets. Therefore, you can add any ticket from business workspaces and service requests from IT workspaces as child tickets.