I use Webex Meetings but I guess it applies for all Webex apps on Mac. You can run/join only one meeting via the app. On Windows you can join many at the same time via the app (you simply have another window opened with a separate meeting).
So in the case of Mac users the only option to participate in several meetings on the same machine simultaneously is to join via browser.
But I guess it's more important for an admin rather than an attendee (unless the latter is going to participate in two at once :D).
