As a "Nebenhörer", FU students can also take courses at the TU and HU and get credits for them at the FU.

How the “Nebenhörerschaft” at TU works
a) Fill out the two forms (1) “Antrag auf Nebenhörerschaft” and (2) “Erhebungsbogen für die Nebenhörerschaft” – you can find them here
b) form (1) must be signed by the lecturers of your classes first! Then go to the dean’s office (in German: Dekanat), where the dean (Dekan) will give you the final signature for form (1)
c) after you filled out both forms and received all signatures you must hand in the form in person together with the certificate of matriculation at the Campus Center
d) They will give you the Nebenhörerschaft and you will receive grades for the courses. Usually it is not a big deal to get all the signatures so you can probably attend your classes.

Unfortunately, they do not have any forms in English, so come and meet me during office hours if you need some help.

How the “Nebenhörerschaft” at HU works
a) Fill out this form – this should be self explanatory since it is available in English and quite straight forward
b) you will need the signature of the lecturers and submit or send the completed form including the lecturer's signature and the department's stamp (and the certificate of matriculation) to „Referat Studierendenservice – Clearingstelle, Unter den Linden 6, 10099 Berlin“