Depends which shifters. I think DA have two ports in them whereas Ultegra and 785 or whatever they are only have a port in each shifter. If you only have an RD you probably don't need a Junction B if you can run a wire all the way to the front junction (Junc A). But if you're using the bar end junction, I don't know how many ports it has - is it enough for two shifters and a RD? Maybe it is with the Y splitter? But if it only has one port, you will only be able to plug in the shifters with the Y and then you're fucked with the RD.
Depends which shifters. I think DA have two ports in them whereas Ultegra and 785 or whatever they are only have a port in each shifter. If you only have an RD you probably don't need a Junction B if you can run a wire all the way to the front junction (Junc A). But if you're using the bar end junction, I don't know how many ports it has - is it enough for two shifters and a RD? Maybe it is with the Y splitter? But if it only has one port, you will only be able to plug in the shifters with the Y and then you're fucked with the RD.