I so don't want to be thinking about this but just to give the benefit of the doubt let's say they are physically incapable of tucking. Even on a "long" toilet. Which, I've for sure seen people in public where I've considered how people accomplish certain things like wiping their own ass.

So if they can't tuck they must either piss first and then sit and shit.... Which I guess is possible. But I'm thinking more like a bucket is used and placed in front, so you piss in the bucket on the floor in front of the toilet while shitting. Then you dump the bucket in the toilet and flush. And hopefully wipe somehow, or maybe use a bidet. I am sure this situation is some people's reality.