AOR1 is the acknowledgement email you receive approximately 4 weeks after the first set of your documents (for most people its their application form and completed checklist) are delivered to IQAS while AOR2 is the acknowledgement email you receive approximately 4 weeks after the next set and hopefully the last set of your documents (for most people this is their transcript and request for academic documents form endorsed by their school) are delivered to IQAS.
For people who send their entire application in a single package (I don't know how they achieve this to be honest seeing as IQAS insists that the transcript must be sent by the issuing institution), they receive only one AOR (or probably AOR 1 & 2 together) and their application is put in line for processing.
Hope this helps.