Assuming that you want the cables to behave in a neutral manner, regardless of the issue of the length disparity the 16 foot cable should be chosen to have negligibly small resistance relative to the impedance of the speaker at all frequencies, and inductance such that inductive reactance at 20kHz is negligibly small relative to the impedance of the speaker at 20kHz.
If those criteria are met, the fact that the resistance and inductance of the shorter cable may be half of what they are for the longer cable will not be significant.
Capacitance is usually an insignificant factor for speaker cables, unless the particular cable has extremely high capacitance, and the amplifier is particularly sensitive to load capacitance.
Contrary to what you may read elsewhere, timing differences resulting from the length disparity will be completely insignificant, because electrical signals propagate through wires at roughly 60% to 90% of the speed of light in a vacuum (which is 186,000 miles per second).
All of that said, my instinct would be to purchase two 16 foot cables, because of the resale consideration that was mentioned, and to eliminate any doubt about whether technically unexplainable sonic effects may result from the length disparity. If you do that, minimize any coiling of the excess length on the cable to the closer speaker, as coiling would affect its inductance.
Regards,
-- Al