Congratulations
Really, a harness style carseat in any position in the back is very safe - rear facing seats are slighlty more safe in an outboard position than a forward facing seat.... It might easier to have newborn outboard & bigger kiddo in the middle still - I always liked having the room on the other side enough to close the door behind me while helping to buckle so as to keep warmer in the snowy months
But, sometimes kids need to be separated a bit for behavioral reasons - it's just easier & safer to keep a bit of distance between them as opposed to having to take your eyes off the road
(On a side note, this is one reason I love seats that harness past 40 pounds, LOL) Anyway, what really matters is that both seats are installed correctly - since any of the back seat positions are very safe, I'd say the primary concern is making sure that they are secure, moving less than 1" & after that it's personal preference