Used when two or more people do something together, often with a nuance of mutual participation or shared experience. Slightly formal or literary in tone.
The whole family celebrated the New Year together.
My spouse and I will attend together.
Alternate spelling without the repetition mark; less common but still encountered.
Reduplication of 共 (とも, 'together') with the adverbial particle に. The repetition intensifies the sense of mutual togetherness.