Monsters that require Double Tuning
So far, the only monsters that require this method of Summoning are:
Optional use of multiple Tuner monsters
Double Tuning is also possible with some Synchro Monsters that do not specify that the monsters other than the required Tuner must be non-Tuners.
"XX-Saber Gottoms" must be Synchro Summoned using a Tuner and EARTH monsters, but it does not specify that the EARTH monsters must be non-Tuners. Therefore, up to 5 Tuner monsters can be used for its Synchro Summon.
If a specifically named Synchro Material is treated as a Tuner monster, it can be used for the Synchro Summon and result in there being 2 or more Tuner monsters used. For example, if "Stardust Dragon" is treated as a Tuner monster, it can be used as a Synchro Material for the Synchro Summon of "Majestic Star Dragon" or "Shooting Star Dragon" in addition to the necessary Tuner monster. If "Red Dragon Archfiend" is treated as a Tuner monster, "Red Nova Dragon" can be Synchro Summoned using 3 Tuner monsters.
In the anime
Visually in both series, this Summoning is represented with the chosen Tuner monsters transforming into rings of fire in place of the usual green rings found in standard Synchro Summoning (although this effect could be unique to Jack given his theming).