What
@HenryAlan said.
Also, the professional sports teams only use the stadium maybe 20 days a year. Having two stadiums makes sense if they both get used for lots of other stuff the rest of the time. For example, White Stadium will still be owned and primarily used by Boston Public Schools. They also say they're developing more programming with other local youth sports, YMCA, Boys and Girls Club, etc. The possible stadium in Everett will host similar things for non-Boston cities, and also concerts and other events. They're all just plans now, but that programming likely adds up to justify two stadiums.