I have the 520 I know that switching between Di2 shift S1/S2 can done on the Di2 menu of the 520.
But it would be nice if we could also assign these modes (S1/S2) to each of the "D-fly channel" (corresponding to Switches A, X, or Y on the Di2 shifters). Currently, we're only allow to assign features like "Next, Previous, Lap, etc.. Would like to see "S1 & S2" features in here too.
But sure if Garmin is able to add these features if Shimano doesn't include these S1/S2 features in their D-fly channel first ?