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.