A tentative contract agreement was reached today between negotiators with the Industrial Technical Professional Employees union and Yellow-Checker-Star, the second-largest operator of taxis in Southern Nevada, but negotiations with a second cab company are continuing, so the threat of strike has not been completely averted. The deal blunts a potentially crippling blow to tourist transportation as two unions representing more than half of the cab drivers working in Clark County were in contract negotiations with the two largest cab groups representing eight of the 16 companies licensed to operate in the county.