As with anything in business -- and in life -- there is a yin and a corresponding yang with outsourcing deals involving multiple suppliers.
Increasing the number of suppliers adds flexibility, and complexity.
Recent studies by PMP Research, among others, show that companies are developing a preference for using multiple suppliers rather than the "one stop shopping" deals that have been traditionally favored in outsourcing.
Among the advantages, according to this IT Week article: "competitive tension" that often yields more advantageous deals. (We think this means suppliers often do some pretty creative tap dancing if there's an ambitious understudy in the wings.)
A new outsourcing model does create a need for new management techniques. An early leader in this area appears to be global banking behemoth ABN Amro, which is utilizing a number of interesting techniques to keep its vendors in line. Among them: having IBM serve as a "guardian," accountable for resolving issues with not only itself, but other suppliers.