For merging SPSS data files with similar cases but different variables, use MATCH FILES. Make sure your case identifier -if any- doesn't contain duplicate values and the files are sorted ascendingly on it. The result contains all cases from both files (like a full outer join in SQL).
Second, MATCH FILES also performs a table lookup as illustrated below.

SPSS data files with similar variables but different cases can be merged with ADD FILES. Make sure that 1) variables and values have similar meanings across files and 2) string variables -if any- have similar lengths before you merge your files.