They can be upgraded independently if only one is available. If you set an ‘upgrade alert’, you will do so by segment anyway. If you look at ‘upgrade with miles’ via MMB, you will again see separate segments. Be aware that upgrading ‘segment by segment’ costs more miles than the entire sector (i.e. YYZ-BOM) in one transaction. If you upgrade - for instance - YYZ-DXB and DXB - BOM only becomes available later, you should call in and ask for the upgrade to be done manually as one upgrade. You may have to argue with the agent to achieve this as not all agents seem to be familiar with the process. It cannot be done ‘retrospectively’ after you have upgraded the second segment through the website! (At least that is the experience of many people on this board)