Cost Factors in Job Part

Is anyone using the concurrent process mode in job entry coupled with additional job parts? We are a plastic injection molding company and sometimes have tools that can manufacture multiple parts at once. I'm convinced adding additional job parts to a job is the solution to running jobs of this nature but the material cost factor and labor cost factor fields are set to type int. Never have a seen material costs of one part be evenly proportional to another. Either way I'm curious if there is anyone else using this how they use it?