It’s usually risky to fly one itinerary on separate tickets. That’s because when you’re on one ticket, if you face a flight delay or cancellation the airline responsible for the irregularity has to get you to your final destination. But if you’re on separate tickets they usually don’t have to do anything more than get you as far as the end of the ticket you’re flying on.
Fortunately — in a world where little seems to go the way of the customer — American Airlines has a policy of taking care of customers connecting on separate tickets as long as one flight is with American and the next is with either American itself or a oneworld partner. And I’ve confirmed that this policy is still in force.