Fusion Tuner Monsters are simply Fusion Monsters that are also listed as Tuner Monsters, similar to Monsters that are Toon, Union or other secondary "Arch-Types". it was already possible to simply make/treat Fusion Monsters as Tuners (such as with Lightwave Tuning or Dragunity Partisan). There are very few non-Vortex Synchro Monsters on YCM and this wiki that use this type of monsters for there Synchro Summon, the most prominent examples being Pitch-Shadow Flash Dragon and Solace Dragon. Otherwise, these are still simply both Fusion Monsters and Tuner Monsters with all the rules and conditions that normally apply to both respective types of Monsters.
Most Fusion Tuner Monsters have the same first effect:
This card can be Special Summoned from your Extra Deck by sending the above cards on your side of the field and/or hand to the Graveyard. (You do not use "Polymerization" and if you use this method of Special Summon, destroy this card during the End Phase of this turn).
"Sea Monster of Theseus" is the first, and currenly only, official Fusion Tuner Monster.