For co-supervised PhD in USA, I am wondering if it is possible to have two supervisor in two un-related field. (For instance, telecommunication, and database). These two fields are about 90 degree dispart (just rough estimate). If this possible? If yes, should the thesis include work from both field?
There shouldn't be a problem with this provided that all of the following are true.
Are you working at the intersection of the two fields? Do you intend to add something significant to both fields?
Are the two potential supervisors compatible with each other? This means mostly in non-technical qualities.
Are you comfortable with each of the supervisors and their work separately?
Is the institution ok with it?
If any of the above are false, however, it might be a bad idea. For example, if you are working primarily in telecom but only need some help in the database area it would probably be better to have one supervisor and seek the additional help less formally.
You don't want two supervisors with different ideas about your progress or about what is important or about when you are done.
Caveat: There may be a few more considerations that you should consider but I can't think of.