First of all, you have to be a student in the Master of Physics course at Freie Universität Berlin (see "Application procedure" above).
Applications to the Double-Degree Program have to be submitted online. In addition, you need to send paper documents (informal application letter, letter of motivation, certified copies of the Bachelor degree showing the overall grade and of the transcript of the first semester in the Master course, CV, a recommendation letter by a professor, and documents that show that you possess good German and basic French language knowledge) to the person responsible for the program (see http://www.physik.fu-berlin.de/studium/master/Doppelmaster_Info). Application deadline is April 30th.
The purpose of the early application to the Double-Degree Program shall help those candidates that are particularly interested in such a Double Degree to decide at which university they should enroll.
If someone applies before April 30th to the Double-Degree Program along with the regular application to the Master program in Physics at Freie Universität Berlin (deadline for the latter: May 31st), and is accepted to the Master program, he/she will know about the outcome of the application to the Double-Degree Program before he/she has to enroll to the regular Master program, and thus can decide at which university he/she would like to enroll.
Successful candidates of the early application will get a provisional admission to the Double-Degree Program, which means they are admitted if their average grade in the first semester of Master studies is 2.0 or better. (If the grade is worse, they have to apply again after the first semester of the Master studies.)
It is possible that you are admitted to the (regular) Master in Physics at Freie Universität Berlin, but do not get the provisional admission to the Double-Degree Program. In that case you can start your Master studies, and either try to apply again to the Double-Degree program one year later (after your first semester in the Master course), or just finish your complete Master studies in Berlin. The reverse case will not occur, it is impossible to get the provisional admission to the Double-Degree Program without admission to the Master in Physics at Freie Universität Berlin.
No problem. Just inform the person responsible for the Double-Degree Program (see http://www.physik.fu-berlin.de/studium/master/Doppelmaster_Info). You may continue with your studies in Berlin, and later receive your Master degree form here.