It's only an SNRI in its pro drug form, which is DXM. The reason it is prescribed with Wellbutrin is because it prevents your body from turning DXM into DXO, which is a powerful disassociative.

Additionally the half-life of DXM doesn't make it a very effective SNRI without changing the body's metabolism. The Wellbutrin is the component that changes metabolism.

I've done a lot of personal research with both DXM and it's active metabolite. And have some undergraduate education in pharmacology. I really don't recommend it.