It seems logical that it would make more sense to do two surgeries at the same time, so why is that only being studied recently in terms of people with scoliosis and other back issues?

It does seem to make sense to perform two surgeries at the same time – it means only going under anesthetic once, one recovery, and so on. However, to do any simultaneous procedures (more than one at a time), the surgeon has to take into account how long each procedure takes, how long complicated they are, and how difficult it might be for the patient to recover.

For example, if the two procedures are quite long, it may not be best for the patient to be under a general anesthetic for that long.