Should I ask a highly involved PhD committee member to be a co-advisor? 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.)
No you should not ask them to be a co-adviser. You personally have nothing to gain but do have something to lose (upsetting your current adviser or other unforeseen issues which I can already think a quite a few more).

Every university/department handles these things differently (advisers, committee membership, tenure, etc...). I would venture that as a second year you probably don't have enough experience to know what is or isn't appropriate for your situation. But, your committee member does (or should). Thus I would say that the responsibility falls on him to initiate this if his work does indeed warrant the title, co-adviser.

If your committee member decides that they would like the "credit" of being a co-adviser and asks that you do this, then you can approach your current adviser as a neutral party and mention to him/her that you have been working closely with "Dr. Committee Member" and he has request that he be considered a co-adviser.

On a more personal note...I had a surprisingly similar situation during my Ph.D. I joined a well-known, well-funded lab, with a high profile adviser who was very busy. So I largely worked with a less senior individual who was an aspiring advisor/PI. How it resolved itself was in my 2nd year, they both decided between themselves that he should be appointed as my co-adviser. They then both approached me with their decision and asked if I would be ok with it. Naturally I said yes, everyone was happy, and it was a complete non-issue.