With no foul play there are only a few ways they would all die together. Carbon monoxide rises to the top, intentional OD is another but I wouldn't put that close to the top, even people who have disregard for their own life won't usually take their pet with them. Gene Hackman just passing of natural causes is a possibility and then his wife was so distraught she decided to join him, IIRC she was quite a bit younger than him so the chances of her also going due to natural causes is slim.
Of course we may never find out if the family doesn't want the stuff released.
I may be in the minority because I never really liked him in the roles he played although I liked a few of the movies.
bob