How to double major at a drama school?

I would not recommend that. Theater is very demanding. People underestimate how much it really takes out of you. Half of my friends back home major in acting/theater/film. Engineering is also very demanding, especially toward the end.

A double major in theater and psychology, or theater and education makes way more sense.

If the school you want to go to doesn't offer engineering, which is generally the case at liberal arts schools, then you can't. You'd have to select a school that offers both programs. It would not be easy.

You can't. Don't expect to be able to double major in anything if you are going for an Engineering degree unless the second major is directly applicable and you plan to take longer than 4 years. And if you are serious about an Engineering degree, then don't get it from a liberal arts school.

You find a university that offers both of your desired majors. The problem is that there's little overlap between those degrees. The only thing they have in common is basic gen eds requirements. So you can expect this to take you at least six years.