How would you treat a patient with two concurrent plasmacytomas whose bone marrow showed no evidence of multiple myeloma and has no other MM defining features?  

While I would want to have a little bit more information before giving any final recommendations (are these extramedullary? What are the specific locations? If sinonasal that would make me think of underlying HIV), in general, I would assume that this represents systemic disease and that if one were...

This is by definition Multiple Myeloma and I would treat it as such with systemic induction chemotherapy followed by transplant. These patients do better than the standard MM oftentimes. IMWG Criteria for the Diagnosis of MM | Inl Myeloma Fn

The short answer, in my mind, is an n=1 approach but almost always in favor of systemic therapy as if this were normal multiple myeloma. Different terms have been proposed for this rare presentation, either "multiple solitary plasmacytomas" or "macrofocal myeloma" (I prefer the latter). My guess is...