Assuming by two installations you also mean two different databases too? If so....
You need to first understand how OSClass names pictures. Every time a picture is uploaded, information about it is stored in the database with a unique ID number (starts at 0 and auto-increments). This ID number is used in the naming of the file. If you have two installations, they're going to be producing duplicate picture file names, since they'll both be using incrementing ID numbers. Obviously, you can't use a single upload directory for two different files with the exact same name. You would first need to ensure the file names are not duplicated by each installation.
