TamTun is a legendary Runeword created by combining a Tam Rune of Ritual and a Tun Rune of Invocation.
How to Create TamTun
You can create TamTun by adding the legendary Tam Rune of Ritual and the magic Tun Rune of Invocation to a two-socketed 
Helm, 
Chest Armor, 
Pants, or 
Two-Handed Weapon.
Overflow
TamTun can not overflow because the amount of Offering you receive from the Tam Rune exactly fills the required Offering of the Tun Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.
Legions
Helltide