Since Rainbow 6 alienated a lot of their fan-base by slowly dumbing down the gameplay, many turned to a game called
SWAT 4. It's a similar affair and you can do the whole campaign in co-op so that's a plus. The only caveat is that it's quite complicated so it may take a while to get used to the 100 different keys you need to use.
Worth checking out though.
Threaded Mode
