It is possible to successfully bond tile to tile with modern technology. Done properly a new tiled installation can be placed permanently over an existing tiled installation.
If the existing tiled surface is dimensionally stable, has the required expansion joints, and is satisfactorily on-plane, this method can be used. Its desirability, however, rests on the user.
The main drawback to this method is its increases in dimensions and the aesthetic appearance differences. This simply means that the new installation may not fit.
For example, if the floor tile is already at a maximum height in a room, it may not be possible to add additional height. Or, if the existing tile in a tub enclosure is already at the maximum thickness allowed for the wall, this method cannot be used.
The aesthetic differences mentioned also include the way the new installation will receive the trim. Generally, the new installation will have to receive some additional trim element in order to be visually appealing.
In the case of flooring, it may be necessary to install a threshold to make up for the difference in height. In the case of wall tile, additional trim elements will need to be used for the same visually appealing reason.
Once the decision has been made to use the tile over tile method, the next question deals with what type of bonding agent will be used. Certain types of organic adhesives, Portland cement mortars, and epoxy mortars can be used. The Manufacturers recommendations should always be used.
In the photos below, we see a typical tile over tile installation on a fireplace surround. This choice worked out quite well due to the damaging effect that the removal of the existing tile posed. The existing tiles were well bonded and structurally sound.
The first step in the tile over tile process is thoroughly washing the existing tile to remove any wax, oil, and coatings that would adversely affect a good bond. The next step, which is recommended, is to rough up the surface with a carborundum disk or belt sander. This step is followed by a fresh water wash to remove the sanding particles.
Next a level ledger was installed that will support the upper tiles until they have achieved an initial set.
The upper existing tiles received a freshly combed "medium bed" Portland cement mortar bed using a 1/4″ x 1/4″ notched trowel. The tiles were back-buttered, placed, beaten in, and aligned properly.
In this case, time was not an enemy and the upper tiles were allowed to cure overnight.
The next day the lower tiles were installed following the ledger removal.
Later the hearth tiles were set finishing the job.