Interdependently is the adverb: it tells you how people or things act when they depend on one another. If two countries grow interdependently, their growth is mixed together — each one’s success leans on the other. If teams work interdependently, they do not finish a job as separate islands; they keep handing work back and forth. You will see this form less often than interdependent or interdependence, mostly in careful writing. Though actually, the -ly only earns its place when you are describing the manner of an action. If you only want to say what they are, the adjective interdependent is enough.