I think it's more a matter of getting the right length for each rider, than a concern about the difference between the two. How tall is your stoker? Does he/she have a single bike and what's the crank length on that?
In theory at least, it should be easier to spin a shorter crank, so going to a longer crank for you, should help somewhat in evening out the desired cadence.
Nonetheless, I'd be more concerned about individual fit, than the difference between the 2.
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