After you enrolled at HU Berlin for the MSc. Statistics program, you can also enroll at FU and TU Berlin. This does not lead to additional fees and should be done to take courses at FU or TU.
At FU, it is called “concurrent enrollment” (“Mehrfachimmatrikulation”). Further information can be found here: https://www.fu-berlin.de/en/studium/studienorganisation/immatrikulation/immatrikulation/zulassungsbeschraenkte/_inhaltselemente/mehrfach.html
At TU, it is called “multiple enrollment” (“Mehrfachimmatrikulation”). Further information can be found here: https://www.tu.berlin/en/studying/applying-and-enrolling/masters-application-enrollment/enrollment/multiple-enrollment/