it is only a guess, but my 306w also has a dual disk tension assy. on it and there is a section in the manual detailing its use as a twin needle machine. in my case its pretty much moot as I don't think anyone makes a *306* needle but for the "standard" machines I don't see it as a problem. I had never heard of dual needle before so forgive me if this is obvious, but twin needle is popular in leather goods, handbags and cowboy boots for example, two rows of sometimes accenting or contrasting stitching.