Wanted to use the the Junction A bar end for a new build and did not want to mount one under the stem, primarily for a cleaner look and the fact my stem was custom painted. This split wire does what it's advertised to to do and does it perfectly. Since the bar end junction box has only 2 ports, the Y-split was needed in order to keep everything hidden.
Do yourself a favor, as I learned the hard way(fault of my own), and get the larger size, as the medium wasn't long enough for my application and internal bar routing. Oh....and get the little Shimano tool for connecting and disconnecting the E-tube wires. Worth every penny!
I added aerobars to my drop bars for a distance event and ran out of ports on even the 5 port junction box A (using single wires). These solved the problem. They function the same as any other Di2 wire.