Artikel 20

If two States have concluded a Supplementary Agreement pursuant to Article 21, the judicial authorities of either State may dismiss an action brought before them or may stay such an action when other proceedings between the same parties, based on the same facts and having the same purpose, are pending in a court of another State and these proceedings may result in a decision which the authorities of the State in which the first mentioned action was brought would be bound to recognize under the terms of this Convention.
The authorities of these States may nevertheless order provisional or protective measures regardless of proceedings elsewhere.