I'm interested in a graduate program, and I've identified a topic and idea that I have in mind for research. However, the related papers have been co-authored by two professors in the same department, who also head the related research labs together, as well. I want to contact them about research opportunities.

How would I go about emailing them? What's the right etiquette?

- Email both of them individually
- CC both of them in one intro email
- Choose one of them and email only them

Any help is appreciated. Thanks!