As is known, there are six major books of ahadith used in Sunni jurisprudence. The Shi'a typically don't treat these collections with such esteem, and instead give preferential treatment to four of their own books of ahadith.

I understand that the Shi'a and the Sunni can have very different ideas about who is considered a reliable narrator of any given hadith, which would account for a significant disparity between the two corpora. In addition, there is a general Shi'a rejection of the idea that any book compiled by fallible men can be considered "sahih", which flies directly against Sunni opinions of the books of Bukhari and Muslim.

However, given that any individual hadith would need to be tried for authenticity regardless of the compilation it is in, why then would Shi'a jurisprudence so prefer the Shi'a compilations rather than the Sunni compilations; if my understanding is correct, each and every such volume would still be treated as a mixture of weak and authentic ahadith.

I am passingly familiar with the Sunni method of determining the health of a hadith, typically by analyzing the isnad and judging the reliability of each individual narrator and whether any links in the chain are missing, as well as comparing it against similar ahadith transmitted via alternate routes. If Shi'a jurisprudence uses a similar method for handling ahadith, with the exception that a narrator considered reliable by the Sunni may be rejected by the Shi'a and vice-versa, then I don't understand the need for such disparate collections of ahadith.

Is there a fundamental difference in the collection and analysis of hadith literature which would necessitate entirely different books thereof? If not (or even if so), why is there so little apparent co-mingling of the Shi'a and Sunni ahadith?