Just get the citations of each document from the Scopus database, export the results into two files, and write a Python script that does a set unionintersection. You can generalize this to any files.
You can export the Scopus results to .csv files, and then use the csv
python module to read the files. You can then have the set operation based on the title of the articles (even though I would also export the DOI and use it). If you know how to program in Python, writing this script shouldn't take you more than one afternoon.