I would think that the ones already tinted would look like limousine tint (5%) if you add to it and maybe be a weird shade? I think you would see the difference. If it were me, I would remove the old stuff and do it all the same with 20%. Its worth doing it right. FYI though, 20% is not legal in some states but 30% is in most I believe? (I have 20% myself)
Sunx is factory tint, slight shade of green. Mustangs came with either untinted glass in base models or Sunx tinted in premium or as upgrades...it looks like my glass was replaced at some point....
my question is would tinting them one shade make them look the same.