I don't think that you are allowed to carry 2 in 1
I have only flown on Jetblue (They only allow a total of 3 dogs/cats in the cabin) and they must be is separate bags.
My Mom has flown US Airways (America West) and same rules.
I guess you could try to sneak them, but what happens if you get caught?
The flight attendents always look b/c the little dog is always sooooooo cute
