This study is researching an experimental drug called REGN7945 in combination with another experimental drug called linvoseltamab, (also known as REGN5458) (each individually called a study drug or study drugs when combined). This study is the first time REGN7945 will be tested in humans. Linvoseltamab has previously been studied by itself (without other cancer drugs) in participants who had advanced multiple myeloma that returned and needed to be treated again after several other therapies had failed. The aim of the study is to see how safe, tolerable, and effective REGN7945 is when given in combination with linvoseltamab, compared with linvoseltamab alone. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug(s) * How many people treated with REGN7945 and linvoseltamab compared to linvoseltamab alone have improvement of their multiple myeloma and by how much * How long people benefit from receiving REGN7945 in combination with linvoseltamab compared with linvoseltamab alone * How much study drug(s) is in the blood at different times * Whether the body makes antibodies against the study drugs(s) (which could make the study drug(s) less effective or could lead to side effects) * If there is any change in pain and cancer-related symptoms, how well people are able to function, and their quality of life when taking the study drug(s)