Step::1 Login to your WHMCS.
Step::2 Go to Clients section and click on View/Search Clients option.
Step::3 Select the Client from the list.
Step::4 Now go to the other actions panel and search for Merge Clients Account option.
Step::5 Click on Merge Client Account.
Step::6 To merge another client, select the option Merge to first client.
That's It !Check out the video tutorial having explanation on the same. You may refer to the blog as well.