Hey,

This is an interesting question - usually the MCAT doesn't get into the minutia so this is a little beyond the scope of question you would expect.

I think you would probably need a total KE across your reacting molecules equal or exceeding the activation energy of the reaction - and KE could be divided between the two molecules (assuming a biomolecular reaction).

However, this kind of gets in the realm of too complicated to calculate in case of the MCAT, because then you might also have to consider how the KE of each molecule moving cancels (gemoetry of collision), maybe do some sort of fancy vector diagram and figure out the overall energy etc.

Usually for chemistry we keep it simple - you just look at the Boltzmann distribution and whatever is above a certain line can react, and what is below cannot. As long as one molecule has KE equal to or exceeding the activation energy, you reaction can proceed and you don't really care or consider where the other molecule is on the Boltzman distribution.

Please let me know if this is unclear.