Generally it is not possible to separate enantiomers with a common C18 column. A simple way to separate enantiomers is to use a chiral column. However by the addition of an enantiomer (e.g. tartaric acid) to the mobile phase or by derivatisation of the sample, it is possible to achieve a chiral separation on a non-chiral column.
Cis-trans isomers are well separated on columns with a high carbon load, like KNAUER Eurospher II C18 P.