First, I'll say that multisession discs are a bad idea. There are numerous issues with using this format. Unless you educate yourself and know the pitfalls and possible problems, do NOT use multi-session.
Second, if it's important, create more than one copy on more than a single media type.
Having said that, there is some multi-session info here: »
www.cgsecurity.org/wiki/CDRWIsoBuster should be able to retrieve the data. I'm fairly sure you need the full version to recover the data. The free version simply allows you to see if it can read the disc or not. With that said, I haven't used it in ages. So someone else may have a better tool or can confirm for sure that the retail version will do the job.