This phase I trial is studying the side effects and best dose of interleukin-12 and interferon alfa in treating patients with metastatic kidney cancer or malignant melanoma. Interleukin-12 may kill tumor cells by stopping blood flow to the tumor and by stimulating a person's white blood cells to kill cancer cells. Interferon alfa may interfere with the growth of cancer cells. Combining interleukin-12 and interferon alfa may kill more cancer cells.