How to ask for a co-supervisor? (master's program etiquette)
No It is not too late. Don't be afraid to talk about it. But as you said, you should first talk to your main supervisor to let him know. He will normally be ok with that. Then, you can talk with the other professor. If the professor is too busy or not interested, s/he will let you know. From my experience, it is generally quite positive for the students to get a co-supervisor for several reasons.