Is it permissible to fast two consecutive months as expiation while there are previously missed days that have not been observed, and is it valid to make up these days after completing the expiatory fast?

Fasting for make-up (Qada') and expiation (Kaffarah) may be obligatory immediately, or with delay, or one may be immediate and the other delayed.

Fasting for make-up (Qada') is immediate if the breaking of the fast was without a valid excuse, and delayed if it was with a valid excuse.

Fasting for expiation (Kaffarah) is immediate if its cause was unlawful (such as intentional killing), and delayed if it was without aggression (such as expiation for accidental killing).

If both expiation and make-up fasting become obligatory upon you: - If both are immediate, or both are delayed: it is permissible to prioritize either one over the other. - If one is immediate and the other is delayed: the immediate obligation must be prioritized over the delayed one. However, if the delayed one was performed first out of ignorance, it is still valid.