I just started my second year of grad school (this spring) in a quantitative field. I have already chosen an advisor.
My advisor is a full professor and extremely busy, but has a ton of resources. I knew this going in, and I chose their lab for a variety of reasons (I have no regrets). One of my committee members is an assistant professor, and they meet with me for an hour every week to help me with projects. I've even started various additional projects with this committee member.
Question: Should I ask the committee member to be an official co-advisor?
I feel bad that the committee member is essentially putting in more time than my official advisor, but not getting much "credit" for it. I know assistant professors need to mentor a certain number of grad students to get tenure, and I want all the time this assistant professor spends mentoring me to actually "count" towards their own career goals.
However, I don't want to make my official advisor upset, or insult them, by asking for this other person to be a co-advisor. (Note: the reason I didn't just ask both of them to be co-advisors at the very beginning of grad school is that the committee member was working within my main advisor's lab, and wasn't even an assistant professor yet.)