You can launch the app multiple times, it's off by default but you can enable it via settings.
You can launch multiple clients straight from the app, you even get a dropup menu to select which account you wish to launch similar to the old logon in the game itself.
The app is still active in the background after you open the game, tabbing back is not an issue.
Besides, if you can still launch games without the app multiboxers would have nothing to uproar against regardless.
I'm sure many of them are using scripts to launch all their clients anyways.