Convertible car seat/ double stroller/ Irish twins

Hey y’all! My husband and I are likely going to be taking in my second cousin when he’s born. Our daughter will be almost 11 months old. We’re trying to decide what convertible car seat to get for each vehicle (so two) and a double stroller! We’ve looked into just getting a second of whatever we have in our vehicles now (Graco 4ever and Safety First Grow n Go) but we didn’t buy the 4ever and we got the GnG on sale 😂 any helpful tips for Irish twins would be appreciated as well!!