A little more exact information is required. You could be referring to both up stream sensors which are close to the manifolds. The two downstream sensors, which are between the light off and main convertors for each bank, or you could be referring to an upstream, and corresponding downstream sensor for one bank.
B1S1, Bank 1 Sensor 1 is the rear bank up stream O2 sensor. B1S2 is the rear banks down stream sensor.
B2S1 would be the upstream sensor for the front bank, and B2S2 is the downstream sensor for the front bank.