Assigning second roles to union members is tough. Where I worked, the bargaining agreement between the company and the union clearly stated that no employee can be assigned work outside of their job classification. Luckily, I had a good relationship with the union president, so we met and hashed out a plan.
The biggest aspect of the plan involved house services. Field employees were asked if they would be willing to be trained in house services work, being paid at the appropriate line rate when utilized. We also asked union employees if they would (voluntarily) be interested in delivering material, picking up debris, managing roadside safety, or driving. I was pleased to learn that most union employees signed up. In fact, it worked so well that an agreement was formalized with the union mandating that all new union employees be trained in their second role duties.