Should I ask a highly involved PhD committee member to be a co-advisor?
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.