Use of 二人 not for people
The counter 人 is usually reserved for people. However, it is not uncommon to sometimes refer to things using 人 as well when you’re somewhat personifying them.
In this particular case, I’ve often encountered 二人 used to refer to two things that aren’t necessarily people.
This is from クジラの子らは砂上に歌う where the character is talking to a personified ship and referring to it (+ himself) as 二人.
Likewise, the next two quotes are from くまクマ熊ベアー where the main character refers to her two bears as 二人とも.