Double Tuning (also sometimes referred to as Overtuning) is special kind of Synchro Summon for which two Tuner monsters are used, rather than the usual one. This type of Synchro Summoning was first seen in Episode 113, used by Jack Atlas when he Synchro Summoned "Red Nova Dragon".
Optional use of multiple Tuner monsters
While "Hot Red Dragon Archfiend King Calamity", "Red Nova Dragon", and "Vylon Omega" are the only Synchro Monsters that require 2 Tuner monsters, Double Tuning is possible with other monsters that do not specify that the monsters after 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 Monster 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.