You will only have to write one Master's thesis even if you receive two degrees. You will have to find a supervisor from one of the two universities of your double degree. This professor will supervise, and guide the student throughout the realisation of the Master's thesis and will be present at the oral defense.
In addition, a reader from a university other than the supervisor’s will be assigned. The role of the reader is to read the final work, be present at the oral defence as a member of the jury and therefore be an evaluator of the work.
The supervisor can therefore be from LSM or from the partner university and vice versa for the reader.
In relation to the master thesis itself, you will have to follow the requirements of the university of your supervisor (on content, methodology, deadlines, calendar,...).
The Master's thesis will be written in English (in Dutch for those who are doing the Master's degree in Dutch at KUL).
An oral defence is compulsory (for your LSM degree, even if the partner university does not organise one), and must be done, usually by videoconference, in the presence of the 2 professors (supervisor and reader).
All information (guidelines, deposit) and requirements (format) for a LSM Master's thesis are detailed on the LSM Master's thesis intranet webpage.
Please inquire about the information and requirements of the Partner University.