Two of the Twin Cities' most prominent healthcare systems, HealthPartners and Park Nicollet, have signed an agreement to join operations, marking the biggest merger in the local healthcare market in two decades. If approved by state and federal regulators, the merger would create the state's second-largest hospital system by revenue, behind the Mayo Clinic in Rochester, and combine two organizations with storied traditions in Twin Cities medical care. Patients shouldn't notice immediate differences, as the affected hospitals and clinics will retain the names of their respective organizations. But the move could presage a new wave of consolidation as Minnesota hospitals and clinic systems realign their services and jockey for market share in the face of ever-rising cost pressures and the rollout of federal health reform.