I read that the front door to the house was left open. Maybe they intended for the dog to get out, but for some reason the dog ended up exposing itself to the killing agent, whatever it was.So why the German shepherd and not the other two dogs still running around outside?
If it was double suicide, I think the wife did it. Under this theory, Gene was failing and she could see that she would soon be facing the grief and loneliness of widowhood, possibly including watching him suffer before the end. She may have decided to skip all that.
I highly doubt he would have gone along with her killing herself, so it would have had to have been her as the instigator. That would make it a murder-suicide rather than double suicide.