Those wheeled ones at Target would probably fit two and then you don't have to carry it around the airport either. You lay it down on it's side in the plane and I would think it would fit under the seat and they can stretch out. Just a thought. I saw some marked down to around $25 at a Target store last week.
http://www.target.com/gp/detail.html...sin=B000EGK6PE
If you read the reviews lots of people used them for airline travel.
There is also someone on here who sells some similar to these that can also be used like a back pack and she will paint your yorkies, etc. on it too! Can't remember who it was, though.