I understand that perfectly parallel would be ideal. Nothing is perfect though and I would bet that all doubles either shoot apart or cross to some degree. Given that choice, I would prefer a little crossing. If the bullets cross at 50 yards then they will only be muzzle width apart at 100 yards and twice muzzle width apart at 150. Getting them to cross at 100 yards would be even better giving muzzle width apart at 200 yards and twice that not till 300 yards. I would rather have deliberately crossing at some reasonable distance than almost parallel but slightly diverging. Diverging paths will only get worse further down range.