If you worked as an actor, you were performing. So, you would want to say something like: "In August of the last two summers, I have performed at the theater." "For the past two years, I've spent a month during the summer performing at the theater." "For the past two years, I've spent the month of August performing at the theater." If you were just a maintenance worker at the theater, you could say: "For the past two years, I've worked summers at the theater." "I've spent the last two Augusts working at the theater." "For each of the past two summers, I worked at the theater in August." Of course, as is always the case with English, there are probably a dozen other ways this thought could be expressed, but these are all good options.

I think it would be a good idea to specify what kind of work you did. You could say: The last two summers I worked as an actor in a theatre during the month of August. OR The last two summers I did maintenance in a theatre during the month of August.