I have a double sind in the kitchen that takes at least 3 hours to drain. It is ithe only fixture at that end of the house. I ran a 50 foot snake just under the drain, the house is 35 feet long. The other fixtures all drain fine except the dish washer which is next to the sink. I replaced the disposal and checked the traps and other pipes directly under the sink. What I am guessing is the vent, about 10 feet from the edge of the house where the majority of the fixtures are, has no plugs for 8 feet; the length of my stick. If you turn the disposal on it is all you can do to hold a stopper in the other sink the force up is so strong. The pipe under the sink is PVC and then switches to the original, 1922, cast iron maybe 2 feet down. The water does not fully drain, about 1 inch worth, from the dishwasher.

If you have snaked the horizontal drain in the wall and still have a problem then you r clog is farther on down the line. Go up on the roof and send a snake down the kitchen vent. Put out enough cable to reach the base and 20 feet more. Good luck, Tom