A collision Wednesday night on Highway 87 in Cooper County sent two people to the hospital.  Benjamin Guillen, 41, of Jefferson City, rear-ended a horse and buggy operated by Mahlon Slabauch, 27, of California, Mo., according to the Highway Patrol.  Both men were taken to University Hospital in Columbia with serious injuries.  The accident was report around 9:00 p.m. near Logan Road.