Somewhat like "Irish Twins", but "Austrian Twins" are two kids born days apart by different mothers with the same father. One mother is the wife and the other is the mistress.
See Arnold Schwarzenegger's dilemma. He had two kids born within days of each other by different mothers...one with his wife and one with his mistress. His kids are "Austrian Twins". (He happens to be Austrian, so he gets the title!)
When your wife and your mistress have your children within a month of each other.
Schwarzenegger fathered children by Maria and his hosuekeeper within weeks of each other. They're Austrian twins.
When a man fathers two children within one year, one by his wife and one by his mistress.