You can set up a case type with second sponsor form such as I-864) then create the case as you would.
There is no difference in case creation process, you just select your primary applicant, primary petitioner and attorney who is going to work on the case.
The second petitioner's questionnaire will automatically show up in primary petitioner and in portal for primary petitioner. Attorney or petitioner can complete this questionnaire.
You can change the contact name by going to the contact manager, searching for the record, and updating the information you need including the contact's name. You can change any detail on the contact record.
You can alternatively choose to change the name in the smart questionnaire as well, which will be only reflected in that case and not on the contact record.