I know the use of WCF in SA is deprecated because it will move to SA Contrib. But until it has, I guess I have to use the support in SA.
That said, I have a problem
I'm not that familiar with SharpArchitecture, but it seems like you have at least two options here:
On the client side, dispose the WCF Service after the first call and new up another WCF Service before making the second call, thus getting a new ISession.
Make the WCF Service smarter about session disposal such that you can keep the session open longer. There are multiple ways to do this and it probably involves a decent amount of new logic in the WCF service, but it should be totally feasible.