Do you have any pix of the drain/vent b4 the concrete was poured?
If not, does the township have the diagrams? It seems odd that they wouldn't require the fixtures to be individually vented. You should really call the township construction offc and confirm that it was done right.
As for the second sink, tie into a vent above ground.
Do you have a sewage pump or below-grade draining? If it's a pump, the vent for the pump should be easily accessible (assuming they vented that).