The effect is an average (not accurate) of both octane numbers that will also depends on the amount in volume that you will add to the blend; therefore the calculated average is dependent on the relative percentage. Anyway, the difference between the calculate number and the real number established by engine tests won’t be far. A good mixing of components is important.

I agree with the previous answer and this means that the knock behaviour of  a 50/50 mix of the two fuels should be close to that of 87 octane gasoline which is often not sufficient for high-performance engines.

I agree with above answers but like to do some addition as follow. You also need to considered density of fuel since its going to affect the mixablity of both fuel and may make fuel heterogeneous resulting  in uneven combustion process. 

In addition to the above spoken, the precise value of resulting RON will deviate from the weighted sum (sum of octane numbers multiplied by volume of both brands). The cause is intermolecular interactions between hydrocarbons in a mixture and sometimes difference may reach +-11 points, depending on the composition of gasoline.

For information, I can recommend the following sources:

1. https://www.researchgate.net/publication/275541824_Calculation_of_Gasoline_Octane_Numbers_Taking_into_Account_the_Reaction_Interaction_of_Blend_Components

2. https://www.researchgate.net/publication/277625185_Complex_modeling_system_for_optimization_of_compounding_process_in_gasoline_pool_to_produce_high-octane_finished_gasoline_fuel

Best wishes.

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