Let's slow down a minute....when, OP, you say 'carry on' item....are you referring to a suitcase/rollaboard bag (suitcase that is 12x22 ish) or a computer bag/small backpack/purse?
The latter can almost always fit into a fully occupied plane, either under the seat in front of you or sandwiched between two bags in the bins. I often take 2 laptops, and still have never had an issue with my laptop bag fitting on the plane. This type of bag is called a personal item in airline lingo (and by many frequent flyers), but many folks also call it a carry on item (because one does, in fact, carry it on a plane)
A suitcase (or large backpack) type bag, aka your carry on (in airline and FF lingo), may or may not fit in the overhead bin, depending upon which boarding group you are assigned to, how many individuals are on the flight, and how many individuals are bringing a carry on onto the plane. Normally, in/around boarding group 2, bin space often becomes limited....so you roll the dice (based upon the other factors outlined above).