Idea to make DF raids less annoying for tanks.

[Rilifane]

There doesn't have to be an awkward moment, just turn on tank stance and pull the boss. Whoever is holding aggro obviously is the main tank.

That's how it's usually done.
Most tanks will simply turn on stance, someone pulls and whoever is holding aggro gets to keep it while the rest either turns stance off or neatly stacks with the current MT. The occasional aggro fight happens but changing the system as suggested would create more problems than it solves.
Splitting the queue for tanks will increase queue time even further because tanks already tend to have the longest queue and it deprives tanks of the option to step in if someone doesn't appear to know what they're doing or dc'ing.