Use first person if you want the reader to directly empathise with the character. A first person character also acts as the narrator for the story, so if you use it, the character should be in a position to know all aspects of the story, even the parts they aren't involved in.
I would advise against using first person where you have more than one main character as you described - if you make them both first person, it could get very confusing for the reader; but if you have one of them being first person and the other written in third person, it will make the first person character seem much more direct, and reduce the impact of the other character. It will possibly also make the first person character seem omnipotent, because since the first person character is effectively the narrator for the story, it will seem as if they know everything that the other character is doing.
Hope that helps.
