My understanding is that a sequential BOV has 2 chambers. With single chamber BOV's you either have the spring loose so the BOV opens easier but can't vent as much air under heavy load, or tight to vent heaps but doesn't open as early (not really good). The twin chambers work by having a light spring on the first chamber and a heavier one on the second so you get the best of both worlds. But you pay for it....
I'm pretty sure that's the right explanation, but if anyone else knows for sure let me know
Brett