Maybe I'm off in the weeds in my conception of how the sync activity takes place, but I think of it as merely creating a mirror image of the data on whichever computer is being used for the sync activity. So, on computer #1 if I have photos 101, 102, and 103 and I sync with that computer, then my iPhone has copies of photos 101, 102, and 103.
However, if I sync with computer #2 with photos 104, 105, and 106, then the iPhone politely obliges and now contains photos 104, 105, and 106. If I go back to computer #1 and re-sync, then the iPhone again will contain photos 101, 102, and 103 - it maintains a mirror image of the files on whichever computer it was last synced with.
My way around this to be able to use both computers for syncing photos is to update my photo folders in both computer #1 and #2 so that they are always identical. If I add new photos to computer #1 then I also add the same new photos to computer #2 - a quick chore since my computers are linked via my Airport network. If I were to have one of these computers at a remote location and wanted to upload new photos to my iPhone, then of course the new sync operation would overwrite the old photos with the contents of the updated folder and its new photos. When I get home then I need to remember to duplicate the folder contents so that both computers are the same again. Or if I want to listen to a new tune on the way home, then I need to accept that the old tunes will be overwritten by the new one, and I will lose the old information until I do another sync with a computer that contains BOTH the old and new information.
Am I thinking correctly? Just to make certain I was not going crazy, I performed a similar exercise a few minutes ago, with my grandson's music on one computer and one of my Canadian Brass favorites on the other. The results were the same as for the photo example above. So that I don't have to maintain identical calendars, address books, etc. I just have to remember to check the appropriate boxes to define the source data for the sync activity, whether it be iCal, Addresses, Music, or Photos.
Dave