The BP2 is actually 2 BP1s in the same enclosure. The A and B circuits operate independently of one another and may be used simultaneously. Only the headset amplifier is shared so both circuits can be monitored on a single-muff headset. Each circuit has its own volume control, signal button, signal lamp, mic on/off switch and sidetone control. The 2 circuits are connected to the rear panel XLRs so a Y-cable or SB1 Splitter Box is necessary for each circuit if you wish to loop through to the next station in that circuit.
The BP2B is essentially the same as the BP2 except that it has two earspeaker amplifiers and a 5-pin headset XLR on the rear. Used with a dual-muff headset the operator can monitor circuit A in one earspeaker and circuit B in the other. The microphone speaks to both circuits.