Now that most airlines except Southwest are charging for checked bags, the full carry-on luggage allowance is being used by everyone. 

So, depending on the size of your plane, where your seat is located, & the order in which you board, you may have to check one of your 2 allowed items at the gate if both won't fit under your seat.

In many cases this will be checked to your FINAL destination, so if you have fragile or necessary items spread out in both bags, you may have to do some very frantic, last-minute arranging on the jetway.

This is in part due to people putting everything in the overhead compartment & also to those inconsiderate people who board first, put their carry-ons into a bin at the FRONT of the plane & then proceed to their seat at the BACK of the plane.

It doesn't save you any time getting off the plane if the person whose overhead bin space you took @ the front has to make his/her way against traffic to the back to get the bag from the only available space s/he could find.

To avoid this hassle:

1) Check-in online ASAP, IE, 23 hours and 59 minutes before departure so that you can get into the earliest boarding group

2) Be at the gate to get into line ASAP

3) Fly Southwest, who lines up the boarding in an orderly fashion so you don't have to fight for a place in line--you know exactly where your place is by number

4)In case you may have to check one of your carry-on items, have any essentials you absolutely must have from the bigger bag (that won't fit under the seat) packed into a smaller bag inside that carry-on so you can just quickly pull out the smaller bag of essentials & relock your carry-on. 

Make sure all fragile items that you can't put into the smaller bag are well-padded. This smaller bag + your one remaining carry-on item that will fit under your seat now constitute your 2 allowable items.

5) Be considerate & don't take others' overhead bin space. Karma happens. :)