The process of combining Clients into Toggl Track boils down to assigning all related projects to the same client.
Let's assume you want to merge two clients: Client One and Client Two
Combining Clients...
Open the Projects page.
Use the Client Filter to make the page display only projects belonging to Client One.
Use Bulk Edit to bulk-assign those projects to Client Two.
From the Clients page, delete Client One.
If necessary, use the Clients page to also rename Client Two to something else.