I am not actually familiar with Tohoku/Joetsu Shinkansen (living in Osaka), but I can tell you that for Tokaido Shinkansen, dwell time is still very much a problem even with parallel platform. For Tokaido Shinkansen, while the door only open for a minute or so, by the time we are leaving a station, the next train (running 3-4 min behind) is already pretty much pulling into the station.

Previously Tokaido Shinkansen only can only runs 14 train per hour (note that unlike Tohoku/Joetsu, Tokaido Shinkansen runs 14 tph over 350km of tracks, and not all trains has same stops), but after some optimization, including a procedure to reduce dwell time by a few seconds, they were able to push that to 16 tph.