The template you used appends one column to another. What you want is more complicated, appending each value in column 2 (#characters) to a processed version of column 1 (#fandom}.
This template does what I think you are asking for. The first code line generates the new prefix from the fandom column by replacing all commas-followed-by-spaces with a period. "BtVS, YYH" becomes "BtVS.YYH". The second code line adds that prefix to each item in the #character list, separated by a period.
Code:
program:
fd = re(field('#fandom'), ', *','.');
list_re(field('#characters'), ',', '(.*)', strcat(fd, '.', '\1'))
You end up with "BtVS.YYH.Xander, BtVS.YYH.Yusuke".