If two languages are decidable, can one be mapping reducible to the other?
That is false. Consider for example the alphabet $\Sigma = \{0,1\}$ and the two languages $A = \emptyset$ and $B = \Sigma^*$.
Clearly both $A$ and $B$ are decidable (by the trivial algorithms that always reject and always acccept, respectively) but it is false that $A \le_m B$.