There are a couple of airlines that actually permit two small pets in one carrier. I have 3 of these Sleepypod Air carriers ([Invalid or Expired Link Removed]). They're fantastic. They have a seatbelt restraint system built-in and a small portion of each end can fold up for tighter (cattle car, back of the bus) under-seat storage.
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Nice carrier. Can you take them out once boarded? Any sedative required? I used to give my dog a pill since he had to travel in the belly.

