First, a new combination: brass/duralumin twinborn. They would make the perfect spy/thief. Walk into a bank, drain your connection so that the guards are less aware of you, sooth away any remaining suspicion, grab a mound of cash, and walk out.
Second, more flying twinborns! Alas, my apologies for the wall of text. I tried to trim things down, but there were a number of objections to respond to.
Deus Ex, if I'm reading you right, it seems like your three objections are lack of control over pulling, the likelihood of collision during flight, and the difficulty of maneuvering. Is that correct?
For control, there are two things to consider. The first is simply Renette (Alloy of Law). She clearly displayed a somewhat fine degree of control over her powers. The second is that a small force applied over a long period of time can have the same effect as a large force applied over a short period of time. Even if a lurcher couldn't control the strength of their pull, they can control the duration of the pull, thereby granting a similar degree of finesse.
Collisions in air would be less damaging than one might think, since it is only the relative speeds that would cause damage, not the total speed. Additionally, since the twinborn can change their mass, they are also changing their mass to surface area ration, which will in turn influence how much air resistance affects them. As long as they didn't keep their mass to surface area ratio the same as the counterweight's, then air resistance would slow one or the other enough that they shouldn't collide.
And finally, maneuverability, yes, that would be problematic if one were to only use a single counterweight. The addition of a second counterweight, however, solves this. The concept is similar to tacking. A twinborn could travel north by flinging one counterweight to the northwest and the other counterweight to the northeast. He could then gain maneuverability by flinging one not as hard at the other, thereby slowly turning the entire effort.
There are two other lesser considerations that are worth discussing. The first is the weight of the ironmind. This method of flying depends on relative masses and the twinborn's ability to adjust his own. If his metalminds are too massive, the solution is simply to use a larger counterweight. However, we don't have even an inkling of how much weight an ironmind can hold.
The second minor issue is that of mental processing power, and the assumption that one would need to have access to a zincmind. However, much as a juggler works by instinct, a flying twinborn wouldn't need to calculate the angels of trajectory, he or she would just need to practice a lot in order to develop the necessary reflexes. Even with two counterweights, this should actually be easier than Vin's horseshoe wheel thingy (fewer items to keep track of, while the need to pull at certain trajectories, and to switch between powers, remains the same).