As the name implies, Sadako vs Kayako (Blu-ray release 2016-12-13) is a crossover between the Ring series and the Ju-on series. We may reasonably expect the screen time is split evenly between the two evil beings – but no, a large part of the movie rehashes the Ring storyline – unfortunate girls watch the cursed VHS videotape and die. There are strange supporting characters added to the plot, but they are strange for no reason and do not enhance the plot in anyway. For example, a teacher is obsessed with Sadako for no reason and is willing to die to see her. Meanwhile, another girl is attracted to the house haunted by Kayako for no reason, and decides to go in, ignoring repeated warnings from multiple people for no reason.
The crossover does not actually happen until the climax, which is severely underwhelming. I think I used the same description for the actual fight in Batman v Superman, but this is orders of magnitude worse here that I run out of my vocabulary to describe it. Both series ran out of steam 15 years ago. It does have one nice idea – upload the cursed video to the internet for many people to see – but the movie basically dropped this idea as soon as the upload is done.