It's not known to the public, at least. I don't know if Margaret had a plan for how many books it would be, but if she did, it's moot now.
As for the trilogy thing, that was a very tentative suggestion that somehow slipped out in the preliminary phases; I don't think it was ever a solid plan. And I never heard any specific mention of a duology. All we know is that BtRW is the first Romulan War novel but is not intended to be the only one.
As for the trilogy thing, that was a very tentative suggestion that somehow slipped out in the preliminary phases; I don't think it was ever a solid plan. And I never heard any specific mention of a duology. All we know is that BtRW is the first Romulan War novel but is not intended to be the only one.