Using the discrete metric on a space $X$, all subsets of $X$ are both open and closed, and therefore any subset $A$ and its complement $A^c$ is a decomposition of $X$ into two open, disjoint, non-empty subsets (as long as $A \neq X, \varnothing$).
2
Umberto P.
On
The sets $X = \{x \in [a,b] : d(x,a) < \frac 12\}$ and $Y = \{x \in [a,b] : d(x,a) > \frac 12\}$ are disjoint, both are nonempty and open in $[a,b]$, and their union is $[a,b]$.
Using the discrete metric on a space $X$, all subsets of $X$ are both open and closed, and therefore any subset $A$ and its complement $A^c$ is a decomposition of $X$ into two open, disjoint, non-empty subsets (as long as $A \neq X, \varnothing$).