I have a Pet Gear Booster seat -- this one:
Pet Gear Medium Booster Car Seat (Interior Measures 10" x 14" ) | Overstock.com It is plenty big enough for two but I wish it was just a little deeper. It stays very secure in the car tho, the way the seat belt fits around it. It also doubles as a very nice bed inside. They seem to like it best of all the MANY beds I have around the house.
If I had it to do again, I might buy the Snoozer lookout, just because it is a little deeper. I have not talked with anyone that did not like the Snoozers. The ones with straps that sort of hang, usually get negative marks in the long run. So -- either the Snoozer or the Pet Gear for stability and pet safety. They are more expensive but they last forever.
BTW if you live in a cold clime, the Pet Gear comes in a heated model! I guess it works great and just plugs into the cig lighter. I chatted with someone a year or so ago who loved that feature. She plugged it in when she warmed up the car and her babies just cuddled down into the warmth.
