If the turbos spin up twice as fast you will produce full boost sooner. To me that seems like an advantage. As well you can run comprabel boost between the two turbos to equal the same amout of boost that is produced by one large single. It will make the same power but it will make it sooner. The only problem is getting the turbos to work together. It is simplier to just use a single turbo. But there is an advantage to using 2 smaller ones.