Carseats

I was planning on buying two Britax Frontiers. Will there be room for another person in between these two seats? Anyone have two frontiers installed in a Prius? This won't be for everyday travel. On day to day, I will only have one car seat, but if we take a two hour road trip, I'd rather take the Prius and save on gas Just wondering if I can fit another small adult in between the two seats.
Thanks.