Alvar AR tags in Gazebo

I am learning how to work with AR tags, and thanks to 3 very useful video tutorials made by The Constructsim I managed to make it work with my robot. Now I want to try to add more than one alvar tag. With the real robot it works, I have 2 different markers with different IDs, however I don’t know how could I place them in the environment in Gazebo? Should I add it inside the launch file? But then how do I define two different poses for 2 different markers?

Thank you very much in advance.