[otrs] Help with Splitting Tickets

Hello Sorry if this is a basic question, but I can't seem to find the answer despite searching high and low... I like to use the concept of Splitting a ticket and changing the customer so that I can assign a customer ticket to a 3rd party (who has a login to OTRS) - that way they are not able to reply directly to the customer. However, it seems like the Split link does not always appear on the Zoom view of tickets in my system. Can only certain tickets be split? What is the criteria for allowing a ticket to be split? Thanks and Regards Mike