Can Kitchen and Bathroom share a common Wall?
You can design a toilet and a kitchen with or without a wall.
I imagine that the component of the kitchen that will possibly go out of your kitchen space (out of wall, floor, ceiling, slab or roof) consists of the sanitary and plumbing items such as the water inlet pipe (20mm in diameter), the floor trap water discharged pipe (82mm in diameter), the kitchen sink water discharged pipe (50mm in diameter) and the cooker ducting pipe.
The component of the toilet that will possibly go out of your toilet space consists of the sanitary and plumbing items such as the sanitary discharged water pipe (82mm in diameter), the basin water discharged pipe (50mm in diameter), the floor trap discharged pipe (82mm in diameter) and the water inlet pipe (20mm in diameter).
All these pipes can be either exposed or concealed inside your wall, ceiling, floor or roof spaces.
In this case, I assumed that your kitchen with a kitchen sink is placed back-to-back against your toilet WC and toilet basin with a wall in between them.
For construction and maintenance purposes, you do not want to have any of those items placed directly back-to-back with each other with their internal piping clashing with one another.
The kitchen and the toilet can have a common wall.
However, you need to determine whether or not your kitchen has a ceiling space, the thickness of the floor slab, and the type of wall that you are using (especially in terms of its thickness).
If your kitchen and toilet happen to be at the first floor (or any other floor having floor immediately below it), in this case, having a toilet WC back-to-back with your kitchen without a ceiling in your room or unit immediately below your kitchen means you will probably be having a visible 90 degree PVC hub elbow below the slab of the room immediately below your kitchen (as a result from the sanitary discharged pipe from the toilet which is deemed ugly in the view of some homeowners).