Are airline seats designed and tested for comfort and safety in the case where a person of size is occupying two seats?
From personal experience, I can say that it is very uncomfortable to straddle two seats. Typically there is a hard ridge in between the seats that is directly below the tailbone. The raised armrests often protrude and contact the spine. The seat belt receptacle is directly underneath the person sitting this way. Furthermore, using two seats does not extend the leg room – so with long legs it is necessary to sit with legs spread apart
That all said, it is better than being stuffed into one seat and encroaching on your neighbor.
The more serious question is whether it is safe to straddle two seats? Could the issues causing discomfort lead to injury in heavy turbulence or hard landing? Is it safe to use a seatbelt extension from one seat to another?
There are more questions than answers on this topic. So if any readers have any relevant facts, please send them my way.