It tests the claim that a group of species shares common ancestors and does not share uncommon ancestors. For example, all apes share a common ancestor, but chimps and humans share a common ancestor that is not shared by gorillas and orangutans.
You can’t test for a tree like structure with just two species. Can you show us a tree with multiple branches with just two species?
There are other pieces of evidence for testing the hypothesis that two species share a common ancestor, such as this evidence: