Nik joined us on "TMZ Live" Monday ahead of Wednesday's daredevil walk over the very active Masaya Volcano in Nicaragua. He tells us he won't be able to fully prepare for the most difficult stunt of his life because his wife is attempting her own dangerous feat over the same volcano!!!

With wife Erendira putting her life on the line above the lava, Nik says he won't be able to get in the same zone that's helped him pull off tons of dangerous stunts in the past, like tightrope walking over Niagra Falls and Times Square.

Instead of getting his mind right for his volcano walk -- 2,000 feet up and 1,800 feet across -- Nik tells us he'll be worried sick for his wife's safety. As for his safety, Nik says he'll be wearing a gas mask and goggles to protect himself from poisonous gasses. What it won't protect him from -- the heat.

While Nik's mum on Erendira's exact stunt -- it's going down Wednesday at 8 PM ET on ABC -- he says it's her most dangerous attempt ever and would set a world record!

Nik also explains where he got the idea for his volcano stunt, and why he's lucky he chose a window seat.