In case you are not aware of it, the Galaxy tool shed wiki explains the tool shed: http://wiki.g2.bx.psu.edu/Tool%20Shed
Just to confirm, you have set up a single Galaxy instance (with a single database on the back-end) with multiple web front-ends - is this correct?I've "inherited" a galaxy setup from someone (details: a contractor set it up but left it unfinished so I'm completing the job while discovering and documenting what was done). The catch: it's setup as a twin galaxy instance with one as a front-end web-server and the other as the job runner.
The Galaxy tool shed is a separate application that has no dependencies on a Galaxy instance (and vice-versa), although a Galaxy instance and a tool shed instance can each communicate with the other, so your question is a bit confusing. A single Galaxy tool shed can be used by any number of Galaxy instances, so if you create a repository in a tool shed instance and upload tools to it, any number of Galaxy instances will be able to access that tool shed and install the repositories from it.