It is possible to have a joint romance with both Britain and America. If the protagonist succeeds in getting both characters to 4 hearts and completes the confession attempt at the garden house before Autumn 2, then the [?] that appears at the student council room on the map selection screen will replace the 'normal' encounter with the 'joint romance' encounter.
The scene begins with America excitedly hugging the protagonist with Britain smiling in the background. All three reveal their attractions to each other and, after a bit of conversation, they agree to a threesome. Once the scene is complete, the protagonist declares her love for both of them and all three agree to enter a ménage à trois.