I have a double lookout carseat for my dogs. They love it and I can even roll down the window and let them stick their heads out since they are safely buckled in. I put the chihuahua and jack russell in it and they have plenty of room. My yorkie is still to small to ride with them in it. When he gets a little bigger, I will start to strap him in it also.
The double does take up quite a bit of room but I have had mine for about 6 years and couldn't live without it. My dogs love to go bye bye.
Here is an example of what they look like. I bought mine on-line but don't remember who from. You can probably find it cheaper if you shop around a little.
http://www.hunterk9.com/site/870877/product/333