How to search for list of papers citing both papers A and B? For given papers A and B, I would like to search for all papers that cite both A and B. I guess one can generalize to papers {A,B,C,...} and find all papers that cite all of them. 

A quick Google Scholar and Web of Science search revealed nothing. Google Scholar has a "cite=..." thing in the URL, which I messed with a bit to no avail.
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 intersection. 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.