Japan has a long cultural history of a practice called Shinju (double suicide between couples or parents and children)
Shinjū - Wikipedia
For this reason, I don't think a Japanese study is the best one to cite for extrapolating rates of double suicide for the rest of the world. The numbers for Japan may be statistically higher simply due to their culture and history. JMO. On the other hand, if this was indeed a double suicide, perhaps the women were studying the type of Buddhism mentioned in the above link "wherein it is believed that through double suicide, one can approach rebirth". But that is a longshot. JMO.
I hope they are soon ID'd and given back to their families.