There is no particular reason why this should not happen. Multiple regression asks a different question from simple regression. In particular, multiple regression (in this case, multiple logistic regression) asks about the relationship between the dependent variables and the independent variables, controlling for the other independent variables. Simple regression asks about the relationship between a dependent variable and a (single) independent variable.

If you add the context of your study (e.g. what are these variables?) it may be possible to give more specific responses. Also, given that all three variables in your case are dichotomies, you could present us with the data pretty easily....there are only 8 lines needed to summarize this:

\begin{table} DV IV1 IV2 Count A A A 10 A A B 20 \end{table}

etc.

There is no particular reason why this should not happen. Multiple regression asks a different question from simple regression. In particular, multiple regression (in this case, multiple logistic regression) asks about the relationship between the dependent variables and the independent variables, controlling for the other independent variables. Simple regression asks about the relationship between a dependent variable and a (single) independent variable.

If you add the context of your study (e.g., what are these variables?) it may be possible to give more specific responses. Also, given that all three variables in your case are dichotomies, you could present us with the data pretty easily... there are only 8 lines needed to summarize this:

\begin{array}{llll} DV &IV1 &IV2 &{\rm Count} \\ A &A &A &10 \\ A &A &B &20 \end{array}

etc.