You had better check with the airline right now to see if they will allow you to take two cats with you. The last time I flew with a cat (about 4 years ago) the airline limited the number of animals to one per person.
Additionally, some airlines have limits on the total number of animals in the cabin and, when I last flew, they allowed only one animal in the cabin - not per person, but only one animal on the flight itself.
As to the carrier, I don't think you are going to be able to find a carrier for a 20 pound cat that will fit under the seat in front of you.
Also, 20 pounds may be over the weight limit for animals, again, check the airline.
This may have all changed, but you need to check on it as soon as possible.
I have always used a soft sided carrier as I've not found any rigid carriers that will fit under the seat.
No one examined the carrier to see if it was "airline approved." It was a fairly inexpensive soft sided carrier we bought at a pet store.
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