Does Clash of Giants count as your damage if you choose two of your opponents to do damage to each other?

In Clash of Giants, the damage dealt is not considered your damage.

According to Rule 417.6.c.1: "The Damage assigned, and subsequently Dealt, to attackers has the defenders as the source and vice versa."

When you use the ability of Clash of Giants to choose two units to deal damage to each other, those units are acting as the sources of the damage dealt to one another, not you as the controller of the spell.