This is from the USBC CAQ's
#5-
If two members are absent and only one substitute is obtained, which absentee score is used?
According to Rule 104a, Item 1, the team captain is responsible for the eligibility of the team and determines whom
the substitute will replace. First, the captain determines whom the substitute will replace and then the absentee
score is determined.
When the team has a limited roster, the process is easy. The captain selects the player the substitute replaces,
and the absentee score of the other player is used.
When a team has additional players on its roster, the captain decides whom the substitute is replacing.
Whoever the substitute is replacing, the absentee score for that absent member cannot be used. According to Rule
105, Item e(2), the absentee score of the player with the most games is used for the other player. If two
absentee scores are needed after the substitute is in place, the absentee score of the absentee with the next
highest number of games is used and when two absent members have the same number of games, the lower
absentee score is used. (See CAQ #9 for cap information.)