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Rather than Y cable, can’t you just wire one shifter to the other, one cable to the junction box and one cable from whatever free port to the internal junction A cable? No soldering needed.
Can do, yes. It's a total bugger to run the cables, particularly if you want one cable running all the way from one shifter to a Junction A in the downtube or seattube inside the bars. It's doable though.
Rather than Y cable, can’t you just wire one shifter to the other, one cable to the junction box and one cable from whatever free port to the internal junction A cable? No soldering needed.
Didn’t think Di2 was fussy about where things were connected, just that they were connected.
I think this is how I wired mine in my Superzero bars (which are awkward as they have a cable channel under the tops, but the only way you can go internal is through the drilled holes near the bar ends)