documentclass[12pt]{article}
usepackage{natbib}
usepackage{multibib}
newcites{apndx}{References}
begin{document}

First paper to cite: cite{X1}
bibliographystyle{ecca}
bibliography{XXX}

appendix

Cite a paper in the appendix citeapndx{X2}

bibliographystyleapndx{ecca}
bibliographyapndx{XXX}


end{document}