How to fix a plumbing line where the kitchen sink water backs up into the garage/laundry sink?

So here’s what just developed and wasn’t a problem a week ago: Any kitchen sink use causes the garage/laundry sink to fill with that same volume of used kitchen sink water coming up through the drain. The used dish soap, food particles (before we started straining/catching it in the kitchen sink drain) all came up with it. Checked the vent from the roof with a snake. Didn’t help. Drano’d both sinks. Didn’t help. Opened up the traps, drained, then tried de-greasing with Borax. Doesn’t look like it helped. The kitchen sink never backs up but the garage/laundry sink does. No bathroom pipes are involved in this situation. The washing machine drain I believe is involved though we just stopped doing laundry at home just in case so I’m not totally sure.

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