I drive a Suburban & this is the best photo I could find of the seat in my car.
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If I'm only using one booster, I keep it in the front seat. It does fill up the seat, but it's not wide enough to use the middle seatbelt latch in the back seat..it wouldn't secure to the seat in properly. If I have 2 boosters in the backseat, my 9 year old son can still sit comfortably between them.
Yoshi (Yorkie) is 7 pounds. Momo (Morkie) is 8 pounds. You can't really tell in the photo, but Yoshi's left foot is actually a little past the center of the booster. Momo always has to be right next to me so she's squished over to my side. There's no way that both of the could lie down in the booster together (unless they were on top of each other). But one could take a comfortable nap with a little blanket.
Hope that helps. I couldn't find a pic of just one of my pups in the booster.