The breed is part of the European Endangered Species Program.
The cubs are now big enough to fight playfully with each other and the brother is already larger than his sister. A competition will be held by the zoo to find names for the new arrivals.
Zoo director Krzysztof Kelman said the birth of the animals for the first time in Plock Zoo was "a huge success", with the possibility of increasing the numbers of the species through a breeding program.
There are only a few hundred Siberian tigers living in the wild, Kelman said. According to him, over the last two years, 23 cubs were born in seven zoos in Europe.