Refer to How to split client and partner records and divorce the client and their partner. This will create two separate client records.
Marry the clients through Tools → Manage Marry. (It's above the Recycle Bin option, in the 'Advanced Records Management' section of the Administrative Tools column)
Enter the name of the client you wish to appear as the primary in the Primary Client field and enter the name of the client record that will be the partner within the Partner section.