Authorities are on the lookout for a man they believe held up a bank and attempted to hold up a credit union in two central Iowa communities yesterday afternoon. The man entered Tri-County Credit Union about 3:30 P-M Wednesday, showed no weapon and demanded money. He fled empty-handed. About an hour later, 16 miles away, the Grinnell State Bank was held up and the suspect fled with an undisclosed amount of cash. The suspect in both cases has the same description: white male, 5-foot-8, dark hair, dark tan and 200 to 220 pounds. Grinnell police say witnesses saw a second man in the getaway car which was a blue late-model Trans Am or Camaro with Illinois license plates.