So, your people would need to live in constant fear of this devil of the underworld. You have to be careful where you go, because the devil could be there and obsess you or at least oppress you. You would have to be very careful what music you listened to or allowed your children to listen to because the other god from Hades could trick your God and sneak in and destroy your family. If someone brought in Ouija Board and your kids touched it, the god of Hades could enter into their soul and your good God wouldn't even notice it, nor could He stop it.
If any event happened, your car broke down, your shoe laces came untied, it would have to be either your God or the other god in Hades as they are in this constant battle . . . some days one wins, some days the other.
Your world would have to be defined by great boundaries . . . made of bricks of "dos" and "don'ts" to protect yourself from this powerful devil. As one great Evangelical preacher said, "God and Satan are like two pit-bulls in my back yard that are always fighting. The one that wins, is the one that I feed the most." They are almost equals . . . the Titans against Zeus.
This good God of yours is also quite finicky. You have to be very careful what you say and do. If you don't walk into His temple every Sunday or if you get mad and say "shit" he is deeply offended because it threaten's . . . well, His self esteem.
Now, my God is so big that it is totally beyond the mental capacity of mortals. I can't even fathom the 14 billion light year distance across our universe and He has to much bigger than that. Satan is a mere mortal and is of total insignificance to my God, although he has been of significance to this mortal world through the fall, but "obsesses us" only if we allow on a psychological level.