To merge two helpdesk tickets into one, click the Merge Ticket button on the main ticket:

Next, search for the ticket you're merging by entering the ticket ID or other ticket info. 

After you find and select the ticket, click the Continue button:

In the next step, select the information from the second ticket that you would like to merge into the primary ticket, and click Merge Tickets:

A confirmation prompt appears as shown below. Click OK to proceed (note that the ticket merge is irreversible):

Once the tickets have been merged a confirmation message is shown:

Depending on how many files have been saved in the ticket, it may take a few minutes to copy all of them to the primary ticket. You will be notified of the progress with notification popups:

Once the operation is completed, a note is added in the main ticket: