I worked at a Wall St. trading firm (albeit not in the trading area), and a lot of the back office/audity type people were required to switch roles every so often, so that any little unforeseen way to steal money somehow wasn't overlooked and "hoarded" by one person. In addition to multiple people having to sign off various things, etc.
IIRC, there were mandatory PTO/vacations required as well, so that everyone's job was done by at least 2 different people during the year.
This was not a legal thing, but required for the firm I worked at. My manager at the time said this was pretty common practice on the street, then. (Early/mid 90's.)
