Monoclonal antibodies, such as pertuzumab and cetuximab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. Giving pertuzumab together with cetuximab may kill more tumor cells. This phase I/II trial is studying the side effects and best dose of pertuzumab when given together with cetuximab and to see how well they work in treating patients with previously treated locally advanced or metastatic colorectal cancer