Some labor unions help you source employees to be hired as regular hires. Some unions will take on the administration of union-specific benefits, like short term disability, etc. At times, some unions may become involved in the performance management or development of your employees.
What they don’t do is hire employees or pay wages or taxes on behalf of your employees. Labor negotiations with a union should not generally constitute a co-employment relationship.
However, if you do have a collective bargaining agreement (CBA), be sure you check it thoroughly before bringing in any kind of contingent workers, as it could be and likely is covered in the CBA. Typically, labor unions are not fond of employers bringing in temporary workers because it can dilute their impact, and those workers could potentially be used in place of striking employees.