Two competing health systems in northeast Tennessee are looking for state approval of a merger that would establish a single operation, which, according to their leaders, is the best way for the two to navigate a challenging health care environment. Wellmont Health System and Mountain States Health Alliance, each a not-for-profit system with a regional footprint, filed Tuesday for a certificate of public advantage, or COPA, from the Tennessee Department of Health. Their headquarters are about 25 miles apart: Wellmont is based in Kingsport, and Mountain States is based in Johnson City.