Just tested this out by battling Boldore (gives 1 atk EV and 1 def EV). Each of the five times my Noctowl was given the attack EV. Therefore I suppose there is a priority for which stat gets the last EV in such a situation, probably following the standard order of stats (HP, atk, def, sp.atk, sp.def, speed).
That's what I tought, but I didn't want to give false information.
I'll pm DNA about this, or another pokemon maniac.
Edit: Wait a minute... 600th post!!!