With the full merger between American Airlines and US Airways happening the 2nd quarter of this year, both airlines now give you the option online when you log in to your accounts to merge your accounts to make the transition easy when it happens.
Miles and any benefits will not be merged as yet though as explained when adding in either airlines frequent flyer number.
One thing to double check on BEFORE you link them together is that your exact name, date of birth, email and address are the same in both programs otherwise it won’t go through. I had to update an email in one of mine and then it worked perfectly.
Both USDM and AAdvantage have bonus promo’s on at the moment for buying miles, so another good choice to build up those point balances if you have some travel planned for the future.