There is no immediate display and how should the watch be able to count the other hand's stroke? If it were sensitive enough, it would count every bump in the road so to speak.
possible. They just need some small acceleration sensors (one for updown and one for forward backward, best a third for left right.) Then the software has to analyse these movements and fit it to the right stroke and count. Should be no big problem to find the actual half strokes, our strokes. Think for now all swim watches just have one of these sensors in and try to predict from cyclic movements and longer pauses (push offs).
Would wish this from the manufacturers, but it seems good enough to sell enough of the actual watches.