Look at Sumiko Ocos speaker cable. I am not sure who else qualifies, but the Ocos cable is not length specific so you can actually get 2 separate runs.
Coiling the cable makes it a crude inductor. If it's a well shielded cable, it's probably less of an issue, but it's one of those audiophile no-nos.
I use Transparent Super cables of differing lengths...I've been told the networks "tune" the resistence, capacitance and inductance of the cable to be identical regardless of cable length, and I sure don't note any problems. I'd be inclined to use the two different lengths....just try not to obsess about it.
Electricity travels about 9 to 11 inches (memory?) in a nanosecond. This would equate to a time shift so far beyond your ability to hear (unequal length)that it is of no consequence. Do what you need to do. An exception would be to get equal length cables in case you ever swithch your system around and center the amp.
I'm new, and maybe naive, so forgive me if I'm wrong. But cable length is not a matter of the 186,000 miles a second, so much as a matter of the load of resistance.
Electrical resistance increases with the length of the conductor. And it decreases with a greater diameter of conducter. If one speaker cable is a LOT longer than the other, then the resistance is going to be different between the two speakers, resulting in different performance in each channel. The longer cable robs power from the signal in the form of resistance. The power available to that speaker is diminished by the added resistance. And we know it's better to have both speakers sound the same. It's better to have both speakers running at, say a dimished "98%" (both cables REALLY long), than to have one at 98% (with a REALLY long cable) and one at 100% (REALLY short).
This effect will be much more significant with small diameter cables.
If you read the published specs on some cables the resistance (acceptance?/capacitance)is very low. Fraction of an ohm. You would have to have great lengths of the stuff on one side to make an appreciable difference. Part of it too is how everything in the circuit (amp to speakers) reacts. Might be ok with some comboinations but not with others. As to cable diameter, most of who use this site are going to use larger diameter cable in any case so worrying about too thin a cable is not a problem in most cases. Most of us could use our speaker cable to boost our cars with.