I know this might sound silly now, but I recognised that your 'objMail' has the member 'Attachements', where extra emphasis is placed on the 's' after attachment. As a naive newbie to the outlook-api I would wonder if it's not possible simply to write this same statement again in order to add another attachment?
If this is not of much help, I would then suggest you to have a look at MAPIEx:
which is a wrapper for the Extended Mapi by Noel Dillabough. It is implemented for both C++ and .Net, which would you enable to use it. Generally it's very well crafted and straight-forward to use.