Two-year community colleges enroll nearly half of all first-time undergraduates in the United States, but to ambiguous effect: low persistence rates and the potential for diverting students from four-year institutions cast ambiguity over two-year colleges’ contributions to upward mobility. This paper develops a new instrumental variables approach to identifying causal effects along multiple treatment margins, and applies it to linked education and earnings registries to disentangle the net impacts of two-year college access into two competing causal margins: significant value added for two-year entrants who otherwise would not have attended college, but negative impacts on students diverted from immediate four-year entry.