Huh. News to me. My MJ-5 had significant hum when used with one pickup only and that doesn't seem consistent with the dual coil notion—or perhaps the coils were side-by-side in the 4-string and there's an asymmetry in the 5-string. In any case, if you do a web search,
this page describing the MJ-4 still shows up despite it no longer being available. And there's bi-coil pickup wording.
Like some others here, I never found my MJ-5's tone very inspiring: kind of vanilla bland. The however-many-coils J pickups and the M-series preamp just didn't do it for me, although that preamp in my M2500 is terrific. The straight J pickups in my JB-5 (no bi-coils) on the other hand are really excellent.