Whenever you add, upload, or set a recipient, you can use the '2nd Contact' field. This name will appear right beneath the 'Name' of the recipient on postage.
In templates, you can use the %COMPANY% variable to specify this data.
If you wish to address both people in a message, such as
"Dear First & First,"
Then you will need to add a Custom Variable that contains the exact words.
Where Custom1 = "Bob & Alice" in your csv or recipient upload/addition.