When travelling with my non-status wife, I usually split reservations on CO about a week out as part of our upgrade strategy, but on one reservation earlier this summer I didn't. I don't remember the exact text, but on checkin I was asked whether I wanted to check for available upgrades. I selected yes and this caused the reservation to be split. There was no explicit indication that this would happen, but in hind-sight it makes sense based on the way CO processes upgrades.