This randomized phase II trial studies how well nivolumab with or without ipilimumab works in treating patients with sarcoma that has spread from the primary site to other parts of the body or cannot be removed by surgery. Monoclonal antibodies, such as nivolumab and ipilimumab, may block tumor growth in different ways by targeting certain cells. It is not yet known whether nivolumab works better with or without ipilimumab in treating patients with metastatic or unresectable sarcoma.