Wrzlprmft's answer is very detailed, and this just serves as my personal experience (since I am doing just this: my Master thesis is supervised by 2 tutors with similar application areas).
If one supervisor gets hit by a bus, you still have a second supervisor who is familiar with your work and can thus give constructive comments on it, hopefully appreciate it and in particular evaluate it. Often, your supervisor is the only person at your university who is actually capable of fully understanding and properly judging your thesis and defense.
This! It is not uncommon for STEM PhD students to do internships or a semester abroad at another institution (this actually happened to one of the supervisors at the middle of my thesis). Having 2 supervisors just gives you the chance to present with the one remaining.
You can benefit from the experience of both supervisors and learn something from one advisor that you cannot learn from the other.
As far as why one would go to the extra trouble of having 2 supervisors think of it as follows: you are able to learn from different PhD students, that can have slightly different fields.
In my personal case, one of them is clearly more into physics and engineering concepts and prototyping, while the other is very knowledgeable in computer simulations, programming, etc. This definitely helps!
My piece of advice after having done such a project is (as in most project management):
- Try weekly/biweekly meetings where everyone is present.
- Send important updates/pdfs or questions via email with everyone as CC. (Keep in mind to address in the email if you need an answer of supervisor X, as people tend to feel less obligated with this approach)
Best of luck in your project.