How did you write the first movie?
Did you format the RW for packet writing or did you use Nero Burning ROM?
Or did you use Nero Recode?
If you used the second option: did you use Disc at Once? Did you finalize the disc?
Sorry for asking, but when you say that "you put it for multisession" was it using the already written disc?
If you made the first recording as "disc-at-once" the program will tell you that it can't use the disc because it is finalized.
You can copy the disc's content to HDD and reformat it (thats the advantage of using the RW at first) and afterwards you can try to use it in multisession.
But as you have so many doubts about disc formats and so on, the better is to follow chef's advice and look at the link.