My wife has often flown with a rollaboard and two carryons that will both fit under the same seat. She's never been questioned on this.

The only time we've had any trouble was from Japan Air Lines in Narita and we were actually legal, it was just I was pulling her rollaboard (this was approaching the plane, not on board--if I've got some of her stuff I always give it to her before actually boarding.) They made us gate-check one bag. I can understand why I was stopped but why they insisted when the mistake was pointed out I don't know.