The overlapping can be done, observing the following precautions:
- Check that the existing tiles are actually attached to the substrate (do not hear the classic vacuum sound).
- After removing any tiles not well attached, it would be good to use an angle grinder (or even better a special sander) to damage the surface of the existing tiles, to improve the adhesion of the glue.
- During the laying phase, apply the double layer both on the pre-existing tiles with grooved spatula and on those to be laid (with smooth spatula). Use a cementitious glue, possibly with a good elastic modulus, and make joints near the existing ones with silicone products.
If you follow these suggestions, probably your outdoor floor will not give you problems in the future.