If you create a return with both taxpayers names as your client's first name, it'd flow through to the return and you'd need to edit the name under Client Information. 

A better way would be to name your return "John & Jane Smith-MFJ" but keep the client name as John Smith (I.e. without changing the client name).  If you ever decide to run some scenario calculations such as MFS, you can then name the new return "John Smith-MFS Scenario" under John Smith and give a custom status of "Do Not File", for example.