The two groups filed a conflict of interest complaint with the district, accusing the transit agency's lead labor negotiator of profiting from the strike.
Thomas Hock works for Veolia Transportation, which provided some shuttle buses during the July work stoppage, at a cost of over $12,000 to the district. The union says that's not fair.
A BART spokesperson, however, called the allegation a "false attack" and said the real amount paid to Veolia was only $500.
If no agreement is reached, trains could stop running as early as Monday, August 5.