In a simultaneous closing, signing and completion arise in a single step.
If signing and completion coincide, the phase of conditions precedent falls away. Nevertheless you should check whether a merger filing or the consent of third parties is required after all, because a premature completion without the required clearance can endanger the deal. A clear completion list is advisable even for a simultaneous closing.
Have it clarified before signing that no clearance is in fact outstanding. A simultaneous closing is only permissible if no regulatory condition applies.