If I've purchased two tickets in advance and reserved side-by-side seats, then the flight turns out to be overbooked, can the airline give my seat to another passenger?"
One way that you can get as close to guaranteeing the side by side is to have it set up as a MEDA or medical case (this is done for stretchers, post surgery for examples). The doctor/health care team that you are dealing with would contact the airline's MEDA desk (sometimes called medical dept but usually MEDA), and then it would be set up as medically required. There is no extra cost involved.
In my over 15 years working for an airline, not once has a second seat that has been booked been taken away from a passenger (and I work for one of the largest airlines in the world). So there shouldn't be an issue, but certainly I would follow the medical route to give myself the extra assurance if I had any concerns at all.
