The top seeds almost always compete at the GP at home as the top draws. The US now has the delicious situation of having 2 top 3 in Ice Dance so the Shibs will compete in two foreign GPs and they will liekly let known their 2 preferences. Most tops seeds in each category usually get to ask for one preference.
Joubert asked for COC last year hoping to complete his set of medals. Alas, it wasn't meant to be.
Are you saying that the top seed can state preferences for two venue locations but only one location can be guaranteed, and the second location cannot be guaranteed?