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 "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 explicitly state that the monsters after the intended Tuner must be non-Tuners.
"XX-Saber Gottoms" can be Synchro Summoned using a Tuner and EARTH-Attribute monster(s), but these EARTH-Attribute monsters can be Tuner monsters as well, allowing up to 9 Tuner monsters for a single Synchro Summon (with no necessity for any of the Synchro Materials to be non-Tuner monsters).
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.
Almost every Synchro Monster could be Synchro Summoned using a Tuner and "Phantom Lord Hide Ride", but this is not a example of double Tuning, as "Phantom Lord Hide Ride" is being treated as a non-Tuner monster.
Monsters which can only be Synchro Summoned by double Tuning
So far, the only monsters that require this method of Summoning are:
|Red Nova Dragon||スカーレッド・ノヴァ・ドラゴン||12||DARK||Dragon||3,500||3,000|