Tiling Onto Tiles

[what]

What Mapei adhesive or other is recommended for successfully for tiling directly on to an existing wall tiled surface?

 

[Spare Tool]

What Mapei adhesive or other is recommended for successfully for tiling directly on to an existing wall tiled surface?

They'll all stick but what you really should be concerned about is what adhesive is holding the existing tiles to the wall, and what's underneath them, only advice you'll get on here is strip em off and start a fresh