As a former UO TM and AP, I have a different perspective on this. UO definitely hired a bunch of former CMs when they opened IOA and WWoHP, and they have a surprising amount of older, long-term and "lifer" employees.
However, I think UO gets the perception that much of its staff is High School and college-aged since these long-term employees get preferential treatment in terms of schedule, position and location assignments. As a result, many of them are in managerial, support and other behind-the-scenes positions, or assigned to low-volume and low-stress locations. They typically get at least partial weekends off and aren't required to work long or late hours during the busiest times.
That means that when the majority of visitors attend (ie weekends and school breaks), you're more likely to see a higher proportion of seasonal and part-time employees in line-level, guest-facing positions. And those seasonal employees are more likely to be high school and college students. My wife is a HS teacher, and despite her school being one of the closest OCPS campuses to Disney, more of her students work at UO and SeaWorld than Disney.
Regarding dual employment, while things certainly could have changed since I worked there, but at that time, it was in the EE handbook you couldn't work at both resorts simultaneously. And it makes sense as to why, especially with part-time and seasonal employees: they're needed when it's busiest and the resorts need full availability to ensure there's adequate labor coverage. Both resorts would likely want to schedule the same employee at the same times, causing a conflict.
We knew there were definitely employees that still had both jobs, it was just official policy at the time to not allow it.