shorter form :

inputfile1=data/in/inputfile1.txt
inputfile2=data/in/inputfile2.txt

[ ! -f ${inputfile1} -a ! -f ${inputfile2} ] && { echo "\n ERROR: Both files not found. \n" ; exit 1 ;}

[ ! -f ${inputfile1} -o ! -f ${inputfile2} ] && { echo "\n ERROR: File(s) not found. \n" ; exit 1 ;}

echo "\n RUN SUCCESSFUL: Both Files found. \n"
exit 1