Lynne (Arnettpartners), biwiring from two sets of output terminals on the amp will not change the load impedance that the amp sees, relative to what it would see in any other connection configuration.
Regardless of the connection configuration, the speaker's crossover network will cause the amp to essentially just see the low frequency driver(s) at low frequencies, and to essentially just see the mid/hi driver(s) at mid/high frequencies. At frequencies that are within the crossover region, that are reproduced by both low and mid-frequency drivers, the amp will see whatever the speaker's impedance is at those frequencies, regardless of the connection arrangement.
Like the others, I don't see any reason not to use both sets of amp terminals. Although of course whether that arrangement will work better than, worse than, or the same as connecting the two cables to one pair of amp terminals, or using either of those arrangements plus jumpers on the speaker terminals, or using a single-wire arrangement with jumpers, can only be determined by listening.
Best regards,
-- Al