If you have a partnership with those who live on the same courtyard - where everyone owns wine, for example - that precludes the need for an eruv. But what if one person owns oil instead? Also, some returning concepts in terms of joint ownership and "yesh Bereirah." Also, if your oil is terumah and your wine is not, and they don't mix, is that enough separation?? Plus - a shituf mavoi AND an eruv hatzerot - one requires bread, one requires wine... but why do you need both, ever? Once the properties have been joined cooperatively, that should be sufficient. The implications for our generation of why we do both boils down to the rabbinic identity of eruv, and the way we prize heritage.