please don't ask me directly. This forum is intended to get users of NeroSDK in dialog with each others. And I am sure there are others reading this forum that can also answer your question. I am just trying to give answers if no one else knows the answer.
Generally, to start multisession discs, you have to use the parameter --close_session in NeroCMD or NBF_CLOSE_SESSION in NeroAPI. This prevents closing a disc so it can be continued later.
If you continue a multisession disc, this flag is not necessarily needed, dependent from the fact if you want to continue the disc again later or not.
To continue a multisession disc, you have to import the previous session that you want to continue. With NeroAPI you can use NeroImportIsoTrackEx to get this session. Then you have to add the imported NERO_ISO_ITEM to the root of your new NERO_ISO_ITEM tree.
With NeroCMD you have to add the flag --import to continue a multisession disc.