You can't lock on the pilot you're flying with, just your teammates. What I'd do is tell your teammates to follow the quest target (right arrow on XBOX, I presume it's the same on PS3) and they'll split up into another group following a different target flight. When you've taken down your first target flight, you can switch to another pilot at the other flight and bring it down yourself. Attacking an airplane yourself is usually faster than letting your teammates try it (at least, this is supposed to be the case
).