For the first time in nearly 12 years, patients insured by UnitedHealthcare of Illinois Inc. will have access to Rush University Medical Center and three other regional hospitals that belong to The Rush System for Health network. The multiyear agreement, to be announced Monday, reunites the state's No. 2 insurer, a division of Minnetonka, Minn.-based UnitedHealth Group Inc., and Rush, Chicago's fourth-largest hospital based on patient revenue, after the two parted ways in December 2000 over a dispute about reimbursement rates and other issues. Beginning on May 1, UnitedHealthcare's 1.2 million customers in Illinois will gain in-network access to Rush's hospital network and the Rush Health physicians group.