SU seat selection
 

Does anyone know if I have an itrn with two people, one E+ and other GM, will I be able to select seats in advance for both?

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Priority seat selection

The privilege is provided on the long-haul flights operated by SkyTeam partners, with some limitations. Please check the privilege availability when buying a ticket.
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PS I called them and L fare doesnt allow seat pre-selection under any condition :-( but its cheap..

No, SU allows pre-selecting a seat only for bookings made in T+ fare buckets regardless of your status with them or other *S carriers.
The only advantage they give to the top tier card holders is waiving a fee for exit seats selection (aka Comfort+ seats) on their long hauls.

But you can access the selection during online check-in, and then be one of the first to select. I have managed to get bulkheads this way. No emergency row though.

True in my case. I am an Elite+ but with a cheap class ticket I was told that I can't choose seats ahead of time. I am just hoping that waving my Delta Elite+ card before the flight will do me some good :)