How to ask for a co-supervisor? (master's program etiquette) I'm finishing up the first year of my master's program, heading into the second starting September. I had proposed a topic to my (then potential, since confirmed) supervisor in October and we agreed to work together. I changed this topic a month ago for various reasons (mostly lack of available data).  My supervisor was supportive of my topic change and throughout this process has been really helpful, responsive, and critical. But he is not really knowledgeable about the topic or the theoretical framework I am using - his helpfulness has come more from his knowledge of the discipline I'm working in (to which I am new) and his commentary on how I present and support my ideas. I feel that a co-supervisor would help focus my ideas and point out possible gaps, flaws, or inconsistencies in my analysis of the data based on their knowledge of the topic.

Is it too late to ask for a co-supervisor at the end of first year? I have a professor in mind who I'd like to ask, and he is someone I already know, but since it's quite late in the year I am wondering if it is appropriate to even ask. (And I'd definitely consult my current supervisor first. He'd probably be ok with it).
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.