This is what I did but primarily because I was getting overloaded with Avios and I really didn't want to earn any more. After learning a bit more about the scheme and then using my AA miles on some trips, I realised that there was much better value to be had from an AA mile than from an Avios for certain regions etc and in many instances the additional charges are significantly lower.
Before, I had thought it was a bit pointless focussing on two OneWorld schemes but now I'm up and running, for me I find that they complement each other well (for now at least), based on my flying patterns, particularly redemptions.
The downside will be that you are earning a smaller number of AA miles compared to Avios per flight, at least initially if you have no AA status so you may want to think about doing an AA challenge to get Platinum status. They will grant you Gold status as you do the challenge and so you earn some extra because of that and then get to Platinum much more quickly.