When a Combined Skill Roll (p. 55 of the 7th ed. Call of Cthulhu Rulebook) is appropriate, the Keeper will state “Roll against (Skill A) and (Skill B).” This means you roll once and compare your result with both Skills, and are successful as long as your result is equal to or under both, or simply one of them as the Keeper requires.
If only one is required and you have History at 45% and Archeology at 50% and you roll a 48%, you succeed because your result is equal to or under your 50% in Archeology.
I also apply this to rolls against both a Skill and an Ability Score.
