TamTec is a legendary Runeword created by combining a Tam Rune of Ritual and a Tec Rune of Invocation.
How to Create TamTec
You can create TamTec by adding the legendary Tam Rune of Ritual and the magic Tec Rune of Invocation to a two-socketed 
Helm, 
Chest Armor, 
Pants, or 
Two-Handed Weapon.
Overflow
TamTec can not overflow because the amount of Offering you receive from the Tam Rune exactly fills the required Offering of the Tec 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