What gas is produced when hydrochloric acid is added to muriatic acid?

Answer 1

Nothing happens. The two are different names for the same chemical. If the two are different concentrations, though, the effect will be like adding water to the more concentrated one. Like when adding water and acid, heat and splashing may occur! Do this safely with proper protective gear, and with the container able to accept any heat generated due to the water + acid.

Answer 2

When hydrochloric acid is added to muriatic acid, the main gas produced is chlorine gas. This reaction occurs because muriatic acid is a solution of hydrochloric acid in water, so adding more hydrochloric acid leads to the release of chlorine gas.