A dual port BOV (blow off valve) is better than a single port unit because it eliminates surge conditions and lag. A dual port BOV uses a valve section that is activated by low pressures, like those encountered during idle or slow speed driving. At higher speeds and pressures, a secondary valve opens to relieve the large quantities of back pressure produced when the throttle is released. Because of the dual valves, the amount of lag during gear shifts is reduced, and dangerous back pressure surges are prevented.
For more info, see Four Things You Should Know Before Installing a Blow Off Valve