There is no risk from the electrical interactions of an elephant-nose and a brown ghost knife (I assume its a BGK that you have already). The main issue is that the BGK can be physically aggressive. The two different fish can coexist in the same tank if 1) the tank is large enough, 2) they each have their own refuge/hiding place, and 3) the elephant nose is the same size or larger than the BGK.
We've found that the best refuges for electric fish like these are PVC tubes that are slightly longer than the fish. You can hide the tube behind rocks or plants to conceal it.
We've found that the best refuges for electric fish like these are PVC tubes that are slightly longer than the fish. You can hide the tube behind rocks or plants to conceal it.