Sole proprietor vs. partnership

Trying to help grandson with taxes. He and his buddy have a small company making T shirts and other items he designs. This is their third year of filing taxes. They seem to have each chosen to file a Schedule C for sole proprietor, listing their expenses and income, etc. identically on their taxes. I would have thought any time there are more than one person involved in a business it would have to be the partnership route. Please advise.