Do you mean why is it not “the two of them” or “the two people” or something else?
The で in 二人で shows that something is done as “via” or “by” something. So, it’s like “as a pair.” Other usages of that で are like, 家族で (as a family) or みんなで (with everyone).
The context that makes this “us” and not something else, is that in Japanese, absent of any context to the contrary, a direct statement like this includes the implication that the speaker is the subject. They just don’t put in pronouns if they don’t have to.
It’s like when you say “Gonna go to the store real quick.” and omit the “I am”
Grammatically, this sentence could be about some other people besides the speaker, but there’s no reason to assume that without context to justify that leap.
EDIT: Also, who translated it? Because the Japanese is in the past tense, but the English isn’t, so unless there’s some kind of reason based on the larger context of the passage, that’s a little strange.