hi Qx57ZtxZcAG3, welcome to CDRLabs
i'm definitely no expert on Multisession (never use the damn thing...too many problems, wierd/unexpected behaviour).
but maybe what you want is, when you put the disc in the drive and start a new compilation, in the Compilation Properties windwow, Multisession tab, of course select Continue Multisession disc and in the Options make sure that Replace files in compilation is not checked.
however i'm not sure that will get you what you want. what this might do is make the old files unavailable and only the files available, not what you want.
you might be able to get around this is if you rename the folder before you burn it. for instance, rename Dir1 to 2006_01_11_Dir1, burn it with multisession and then rename it back to Dir1. make sure the Options has Remove deleted files from compilation unchecked
. so that when you burn 2006_01_12_Dir1 to the multisession disc, 2006_01_11_Dir1 won't disappear from it (after all, when you burn 2006_01_12_Dir1, the 2006_01_11_Dir1 isn't on the HD anymore).
maybe you should simply uncheck Refresh Compilation when importing session, but i'm not sure what that will do.
try it out and let me know, i'm curious as to how it behaves with the various settings.
search the forum, there was some discussion about Multisession and how to use it a while back.