It's unlikely that the sterns actually tow out unless there is play in the beam to hull connection. The back beam is too close to the transom for that. Although pressure from the mainsheet bending the beam up, could possibly pull the transoms closer together slightly.
Is it toe or tow?
If you had a boat that didn't flex at all with a rudder system that had zero play then parallel rudders may work best.
if you can adjust you rudder alignment whilst on the water, just take the boat up in to irons adjust 'em and sail on various points of sail. Go by feel to find the sweet spot.