My vote is on low octane. If it were debris or gunking, it would be a hell of a coincidence for it to affect both bikes simultaneously. Water, which sits at the bottom of the tank, gives you an instant (and most likely permanent) engine stall.
Although I love my 800, a bike with a thirst for 95RON (or 91 if you had the dealer adjust it for that) can be a liability in the second and third world, where fuel is often considerably lower than that.
Low octane gives you funny noises under load (pinging / detonation), shit power, and the engine will struggle so much to adjust the ratio right you'll get either massive exhaust overheat or thick palls of smoke out the back (depending which way it goes with mixture to try and compensate).
"It's cunnilingus and psychiatry that's led to this" - Tony Soprano