In this talk, shortly before his death, Owsley Stanley, the famous 60s LSD cook, claims that mixing equal parts DMT and DET and smoking the mixture creates a DMT experience that lasts 2 hours.

Has anyone tried this? Is there any validity in this claim?

idk, but DET has been scheduled C-I since the Controlled Substances Act was enacted in 1970; there is no natural source for it,
it's like a unicorn in the synthetic chem world.