One should accept an answer if one is satisfied with the provided answer. In my opinion, in many practical cases, if one gets a satisfying partial answer, but for some other reasons is also interested in the full answer, then it is better to ask a separate follow-up question, than to leave the existing question open (after all, the op has gotten what he wanted).
You have not told what you want to achieve with "two levels of acceptance", so let me just suggest some work-arounds that you may find helpful:
if by a partial acceptance you want to say that you have found the answer the best, just say it in a comment,
if by a partial acceptance you want to reward the author with points, just do it manually,
if by partially accepting an answer you want to make it easier to find your question among other questions having partially accepted answers, then create a tag "partially-answered" and attach it to the question.