Is it possible to retrieve data from multiple files while using Processing?

Yes.  You can use loadStrings() and store each file in an array; though you might want to do additional parsing to split each line on the commas...  And you might also want to check if there's a library for handling CSV files ;)

 Also how can I retrieve the data values from file B corresponding to a data field in file A?

Probably depends on the format of the data.  If you know that, once parsed, the location of the corresponding data in the two arrays will match it should be easy: just use the same indices to retrieve the data from each array.  If you don't have this guarantee you'll need some kind of unique identifier to match the data in the two files and use this to access them.  And you'll probably want to use a more advanced container type - possibly a hashmap -  though I'll leave it to the experts to advise on that.