From Westone who can explain better than I...
Dual Bore
Typically, multi-driver in-ear monitors have a single-bored sound port, meaning the output from each driver will acoustically sum inside the earpiece housing. With Westone's Dual Bore technology, the high and low frequency sound components are individually channeled through two passages in the sound port. This allows high and low frequency material to sum within the ear canal, emulating the more natural phenomenon of live acoustic summation. The result is a more convincing transition between frequency ranges and is audibly more natural.