IMO I wouldn't do it @ the same time to make 1 water change. I would just ur levels may be way off. It's ok to change from 1 salt to another slowly but to mix both I wouldn't do it.
Ken he's been using both salts... I don't think it matters at this point.
Matt, make a water change with what u have of one salt brand then on your next water change use the other brand. Just as you been doing just in smaller valumes.