Tufts Medical Center and Lowell General Hospital are joining forces to form a new health care system they say will offer lower-cost medical services from Southern New Hampshire to south of Boston at a time of changes sweeping through the industry. The alliance would link a 415-bed Boston teaching hospital that has been scrambling for a niche in the fast-consolidating market with one of the state's largest community hospitals. Lowell General, which two years ago took over a crosstown rival, Saints Memorial Medical Center, has 434 beds on two campuses. Under terms of a tentative agreement approved by both boards, each nonprofit hospital would continue to operate independently under a new parent organization, which has yet to be named. Other health care providers could eventually be added.