If you're looking to maximize safety then the least protected child goes in the most protected spot (meaning the middle). So since your older child is forward facing they get the middle seat and the rear facing child gets the outboard seat (rear facing offers significant side impact protection not offered by forward facing).
However, depending on your vehicle it may be impossible to install two seats side by side and get acceptable installations. When doing three across or two directly next to each other both seats need to be independently tight (sometimes a seat will seem tight then you move the other seat and suddenly the first seat is actually quite loose). So install one seat, use chalk to mark where on the LATCH strap/seat belt it was when there is less than 1" of movement at the belt path. Remove that seat. Put the second seat in and make sure it doesn't move. Then put the first seat back in and make sure it's tightened to the chalk mark. If you can't get the second seat in to the chalk mark then you'll need to not install the two seats side by side.
Also, (this may be common knowledge, but I've seen a few people who did not know this so it's better to say just in case) very few cars offer LATCH for the middle seat, so make sure if you're installing you either use seatbelt or confirm that LATCH is an option for the middle. And if it is allowing you to "borrow" LATCH anchors from the side never attach two carseats to the same LATCH anchor as it can't take the force of two seats at once.