I do not believe there is a way to only change the Service Export directory. There is an help article that suggests that you can, but I have tested this on a few versions and it does not work. That help article will be adjusted in future releases. The only way I'm aware of to change the Service Export directory is to change the file://web: storage class path.
Just as we would advise against sharing an explicittempdir, I do not believe you should share the same file://web: storage class path between multiple nodes or applications as this could lead to issues. For example, if users on different nodes try to upload a file with the same name at same time. This is a simple use case, but during high load it could become problematic. There are likely other issues that could arise as well, though I'm not certain what all the possible issues are.
I would instead suggest using the same shared root folder and making different sub-folders for each node / application (for example C:\SharedDirectory\App1Node1, C:\SharedDirectory\App1Node2, etc.).