I have a mix signals (2 signals with different dc offset). How to recover another dc value which has been buried by the stronger dc value?
In general, to be able to separate two signals, you need two different mixtures of there signals. It's very similar to solving a system of equations with two unknowns. You need two equations if you have two unknowns. If you simply have a signal which is a sum of two other signals, you cannot retrieve the individual components.