For a Video DVD you may never have multi-session (period.) The format of a Video DVD is cast in stone and must be a single closed session.
Also, multi-session recording can be unreliable. I'd just put the two or more files I want on that DATA DVD and burn them all at once.
I have nero7. When I record a film to a dvd-r (which on a fresh un-used disc is ok) and then when I try try to record another film to the same disc it comes up with this disc can't be written too or it's full or this is a read only file or something. Nero comes up with multi session etc options and I have tried every avenue as far as this is concerned. So can anybody tell me what in the name of buddah (because sometimes you need the patience of said deity) is going on?