Two Omaha-based health insurance organizations have formed a new partnership. Midland Choice and Midwest Select announced their merger today. The companies are “Preferred Provider Organizations” or P-P-Os, which means if you are insured by them, you must use one of the doctors under contract — and go to one of the hospitals on their list. The new P-P-O, called “Midland Choice”, will serve 600-thousand customers in Iowa and Nebraska.Linda Sufficool is president of Midwest Select, which will give up its name in the merger. She says those who buy insurance from the new company will be able to choose from 10-thousand physicians and 250 hospitals.Under most preferred provider plans, patients are required to use the doctors chosen by their employer, but as of January, 2001, patients in “Midland Choice” will be able to choose their own physician — from the list of 10-thousand who’ve signed a contract with the company.Mercy Hospitals in Des Moines, Sioux City, Mason City, Clinton and Dubuque have signed contracts with Midland Choice to provide patient care. Most of Iowa’s 110 hospitals will be covered by the network