United allows 4 pets in-cabin flying economy. Additional stowaways can vary in number depending on the particular United partner airline, so make sure you Hoomans call ahead. A $125 fee per pet per flight applies, and Fluffy must fly the friendly skies in a carrier underneath the seat in front of you. Check out the United In-Cabin Pets Policy for exact seat measurements.
US Airways permits 6 pets in the cabin per flight and does not allow any pets under the plane in cargo (hurray!). A $125 fee applies for your high flying pup. If you’d like to bring a second pooch on board, you must purchase another seat on the plane, with no additional fee imposed. Dogs must fit in a carrier underneath the seat in front of you. The US Airways Pets page has all the deets.