I have a double sink in the kitchen (one side has a grabage disposal). The bathroom that is downstair is located directly beside it. When my kitchen sink clogged up the food from the grabage disposal came out of the bathroom sink. The plumber fixed it and explained that there was a t connecter that connected both sinks and it made it difficult for anything to drain. But when I got the invoice he claimed he pulled back a large thing of hair. (note I do not do any hair in that bathroom and again it was the food running out. My father explained that the kitchen and the bathroom shouldn't be connected to drain. Asking your perfessional opinion. They are trying to charge me because they claim it was the hair and not the connecter.

Listen to your father in law. In addition to all the grease and soap that goes down the drain from your kitchen sink we now have garbage which is practically solids.
Add to tha6t the grease and hair( yes! Bathroom fixtures do collect hair) you have a blockage just waiting to happen.
My code mandates a separate drain for the kitchen, ( if you have a septic tank it also requires that you pipe both the kitchen and the washer to a grease trap before entering the septic tank)
The county recently increased the drainage size from 2" to 3" to add better drinage.

They are trying to charge me because they claim it was the hair and not the connecter. I can't say that it wasn't hair that they pulled out but I can tell you that any plumber worth his salt would bust his chops to find a way to avoid the connection you describe. Good luck, and please keep me informed. Tom