You really need to ask the MOC's office because they vary on what they recommend or allow. Our senators compare lists and will only give one nomination per candidate and warned you NOT to try to get around this by listing one academy first with one senator and a different one with the other senator. They said if that happened, when they compared the lists that they would still only give you one - and it might not be the one you would have preferred!
Our congressional district on the other hand is less competitive and allows you to list (and receive) nominations to more than one academy. They said generally if someone is strong enough to receive a nomination, they'll receive them to any they ask for, but of course would still go in order of merit so they'd get it if they are one of the top 10 strongest for that academy, irregardless as to whether they received one for a different academy.
It's definitely a gamble. Generally once the nomination is received, you can't change your mind and ask for a nomination to a different academy because they've already been awarded to other candidates. However, a lot of interviews aren't until late November or even December, and there's probably a good chance that you'll know if you have an LOA by then. Many MOC's will allow you to change your rankings up to the interview date - especially with a good reason like having received an LOA.
Overall I think the general recommendation is to list the academies you want, in the order you want, for all the MOC's (ie they should all be the same), unless the MOC's themselves recommend otherwise. If you don't, you could get caught up in MOC's that compare and get asked how you could consider USNA your first choice on one and want USAFA more on the other! However, you should definitely make clear in your essay and your interviews that you would also like to receive a nomination to your number 2 choice if at all possible. A lot of this will depend on how competitive your area is as to whether or not it will be possible.
