How can diastereomers be chiral?

Given this definition, it is fairly easy to suggest examples. Now take $\text{cis-butene}$ versus $\text{trans-butene}$. The connectivity is the same for each isomer: $C 1$ connects to $C 2$ ........to $C 4$, however, these isomers are manifestly geometrically different.
If we go to an organic molecule with 2 stereocentres, which I acknowledge was the subject of your question, there could be $R , R$ versus $S , S$, which are clearly optical isomers; but there could also be the other pair, $R , S$, and $S , R$. Sometimes $S , R$ is the same as $R , S$, i.e. in a symmetric meso compound. We would say that $R , R$ and $R , S$ share a diastereomeric relationship; i,e. same connectivity but different geometry.