May I play both Agendas "House of Dreams" and "Knights of the Hollow Hill" and therefore start with the Gread Hall locatin in Play?

Nope.

"Before you start the game, you may choose a
single agenda and place it beside your House
card to gain its benefits (and drawbacks) for the
entire game."

Ok, After rereading the House of Dreams Agenda, it's clear, that i can't bring the Great Hall since it is non-unique.

But would it work with the Iron Throne, that is an in House unique Card?

Ok, so i can only Run One Agenda.

That solve the Problem.

Thx