Since finishing The Dragon Reborn and experiencing Nightblood's abilities in the SA, I've been wondering what would happen if Szeth and Rand crossed blades. Here are the conclusions I have arrived at.
1.) Saidin/One Power is Investiture. In this instance, Callandor acts as a conduit for the One Power. When the two blades touch, NIghtblood draws Investiture from the True Source. Callandor is not destroyed, but instead of Rand drawing it's power, Nightblood does so. However, Nightblood may destroy the Sword that Cannot be Touched. If that happens, well, then Rand is dead. But, since there is no Cog/Spirit Realm in WoT, I don't think this is would occur.
2.) Saidin and the One Power are not Investiture. In this instance, I believe that Callandor would be destroyed, as there is nothing else for Nightblood to feed on.
A few other things.
The former theory is assuming that Tel'aran'rhiod is WoT's Cognitive Realm and the True Source is the Spiritual Realm. Which semi-works with the way the World of Dreams and the CR behave.
You can die in both.
You can channel/Surgebind/use other means of Investiture in both
You can move freely if not easily between both.
Things are present in CR/World of Dreams that are not present in the Physical Realm.
Thoughts, Sharders?