Generally yes.
If the AA is a feeder to a BA longhaul, then BA would typically be the validating carrier anyway.
If AA is feeder to an AA longhaul trans-Atlantic, then it would typically be AA. BA may be selected as the validating carrier under the joint business, but usually only if they're on the itin as an operating carrier (AA op by BA).
Generally, the most significant carrier on the itinerary is the one whose ticket stock is used, similar in a way to how the baggage allowance is calculated on a mixed carrier itin.
As a travel agent, I get to play with validating carriers all day. AA/BA are common rated so it doesn't make a difference from the TA POV. On mixed airline/alliance itineraries though, I can often find ways to dump fuel surcharges or the new LH Group GDS fee, etc.