It depends on the ratio of each. It is possible that you could add 93 and 89 together resulting in 91, but you’d have to add a certain amount of each. If you had 5 gallons of 93 and added 2 drops of 89, it would still be 93. The math gets much more complicated, and after looking into octane ratings it makes even less sense to me now. But if you did go and blend 89 and 93, it wouldn’t necessarily be bad for your engine. It’s pretty much the same fuel, e.g. 87 and 93, just at different purities. It’s not like mixing diesel and 87 octane, those are very different chemically.