I recently had my kitchens remodeled including adding dishwashers and garbage disposals (in both units of a duplex). The waste from the kitchen (appears to be the garbage disposal) backed up into the bath tub.

What should I look for?

I recently had my kitchens remodeled including adding dishwashers and garbage disposals (in both units of a duplex). The waste from the kitchen (appears to be the garbage disposal) backed up into the bath tub.

What should I look for?

A clog downstream from the bath tub. In would snake from the bathroom roof vent or from the tub waste itself,(see image).
If you snake from the roof vent put out enough snake to reach the base and 16 feet more.
To snake your tub remove the two screws holding the chrome face plate and pull up and out on the linkage and plug. Feed your snake in through the opening. You will run into the return bend of the trap about 18 inches down but crank and work your way past that yoo should only have 6 or 8 more feet to run out. Flush with a tub of hot water. Good luck and thank you for rating my answer, Tom