Uneven speaker wire runs

It's not the delay that can make a difference but other factors such as capacitance and impedance that can add up in cable length. I wouldn't recommend using 21' speaker cables at all. If you must, use low capacitance (teflon dielectric) 12 gauge wires, or more. You can also look at Ultimate Cables which is willing to do custom orders.

If you are going to the trouble of bi-wiring (dubious from an engineering perspective if this will make an audible difference given a low impedance SS amp driving your speakers) then I would go for exactly the same length wires to be safe (after all you are spending a lot of money - so why not spend $200 more and eliminate any doubts/worrries)

Keep the wires the same length, use 10awg and if you bi-wire make them a true bi-wire. Two seperate runs. Impedence, inductance and capacitance needs to stay consistant and to do this the wire needs to be the same length. The amp has a different effect when the variables of the wire changes. Two 10awg speaker wires from two differnt mfg's will most likely sound different but the bits and pieces may travel at the same speed with some of the bits and pieces getting smeared along the way. It's like comparing the speed of your thoughts to your vocals with a major hangover to the speed of your thoughts to your vocals without the hangover. Somehow there is less smearing when sober. Believe me, you don't want to start talking about skin effect.

Just a point of order, audio signals don't travel at anywhere near the speed of light. Only light in a vacuum travels at 300,000km/sec. Even light travels at less than this depending on anything (eg. gases) it must pass through.
The speed of an electric current is dictated by various things including resistance; lightning through air only travels at around 160,000km/hour, not even close to lightspeed.
Audio signals travel by electrons being "bumped" along through the copper or silver crystal lattice, at nothing like lightspeed.
My view would be that, if you had one 1000 foot speaker cable and one 10 foot, you would notice an obvious timing error, a delay if you like. Now, 20' versus 10' isn't quite the same degree, but the effect will still be there at a much smaller delay. I don't think I could hear such a delay, but some keen-hearing audio buffs might.

Carl, do the math. Even at the slow 160000km/hour speed of lightening, one wouldn't detect the difference in arrival (or delay) between 1000ft and 10 ft cable. Don't you just love mixing units of measure?!!