The BP1SL is a modified BP1 that can switch between two circuits. It can communicate only with one circuit at a time. It does have a unique feature allowing both signal lamps to work at all times. If you are using circuit A and someone is trying to contact you from circuit B, the circuit B signal lamp will light, and vice/versa. The circuit connection XLRs on the back are both female, so you cannot loop-through to the next station as you can on a BP1, without the use of a simple Y-cable or SB1 Splitter box.