A RF car seat doesn't work in the middle of the second row bench. It has plastic parts that cover the hinges. A RF seat hits on those plastic parts. At least a Radian & Marathon do, I'd think all RF seats would. In an accident, that plastic could break and make the installation loose. A tech from this board worked on it and confirmed the problem. That's the only spot that doesn't hold RF seats. FF seats work great there.
So...I'd put them both outboard. You should have plenty of room for the front seats. I do find the seats to be squishy so after a couple days you'll probably need to tighten a bit more. A small adult should be able to fit between the seats in a pinch. You could also load through the hatch if you really needed someone back there.
Just so you're aware, sometimes an older child can use their foot to reach the button that flips the smaller part of the seat (the child on the passenger side). My girls never did it and they rear faced until about 4. My daycare figured it out. I'm sure I could've worked with her to stop, but I was lazy and just moved seats around. I think she could do it in the Marathon but not the Radian. Luckily, the seat doesn't actually flip when a car seat is in by LATCH or seatbelt. It just makes a fun noise and is probably super bad for the motor on it.