Last year the same thing happened at my school and so they gave a joined dux, which is funny because it was obvious the science guy was going to get a higher mark than the humanity guy.However in instances where there's a discrepancy of some kind (last year it was due to one person topping 4 subjects lower in the scaling scheme of things versus someone topping 3 higher scaling subjects) then they do it on estimated mark.
(And its given out prior to externals)
this in my school too.Whoever gets the highest UAI... now ATAR
same. i think the best at my school is 99.70Whoever gets the highest UAI... now ATAR
dux of the school is given after the results are released.Highest ATAR. The DUX is given the year after so it can reflect the highest results.