I've not canceled my CO card, or for that matter any of my Chase cards:

--Sapphire Preferred (Aug. 2011)
--CO OnePass (~Dec. 2011)
--UA Explorer (~Feb. 2012)
--BA "100k" (that's really 50+25+25k) (April 2012)

All were approved, though for one or two I may have had to move CL around. I'm not sure if I waited 60 days between the Sapphire and the OnePass, but it was very close. The others were pretty close to 60 days as well.