Thermo-Alchemist’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered. Players can cast spells and activate abilities (such as Thermo-Alchemist’s own activated ability) after the triggered ability resolves but before the spell that caused it to trigger does.

In a Two-Headed Giant game, Thermo-Alchemist’s activated ability deals 1 damage to each of your opponents, resulting in the opposing team losing 2 life.