My advice is to wait until your older dog dies to bring an Akita into the household. Especially since your current older dog is a female, a female Akita pup will take full advantage as she grows to boss your current dog around. Won't matter that you bring in an Akita as a pup. No point in subjecting your old dog to abuse, which I am betting she would surely get from a female Akita. As for keeping them separated...that's no way to live, for you or the dogs. I'm sure you don't want to find that a growing Akita maims or kills your older dog, which isn't a certainty, but is a distinct possibility. It's not worth it. Wait on the Akita pup. I live with two Akitas, an older male and a younger female. Even though they're opposite sex (and altered), the younger female decided from the get-go, as a little pup, that she refused to play second fiddle to any other dogs, including my male who was 5 years old when I brought her (the pup) in. After some bloody & very dangerous fights, I decided to keep them permanently separated, which is how we now live. I love them both to pieces, but again, wouldn't recommend that anyone set themselves up for such a thing knowing in advance what is likely with same sex Akita pairings w/other dogs. It can even happen with opposite-sex if you get an extremely bossy female, as I have, and I wouldn't dream of putting an Akita with another same-sex dog. I understand that some very experienced (and very lucky!) Akita owners have been able to do it, but I sure wouldn't advise it for any but the most experienced Akita owners, and only then with only the most docile Akitas.
