Here is how you get the last baseline (known as the 'basis') of a component in a workspace or stream.

// Fetch a handle to the last baseline (basis)

ITeamRepository repo = <repo_of_the_stream>;

IWorkspaceManager manager = SCMPlatform.getWorkspaceManager(repo);

IWorkspaceHandle workspaceHandle = <this_is_the_handle_for_the_stream> ;

IWorkspaceConnection wc = manager.getWorkspaceConnection(workspaceHandle , monitor);

IComponentHandle component = <this_is_the_component_you_care_about> ;

IBaselineHandle basisHandle = wc.getComponentInfo(component).basis();

// Repeat the above for the other stream.
// Compare the two baselines (it sounds like you know how to do that already though)



//Fetch the full baseline (if you want the name of the baseline etc, etc.)

IFetchResult fetchResult =  repo.itemManager().fetchCompleteItemsPermissionAware(Collections.singletonList(basisHandle), ItemManager.DEFAULT, monitor);

IBaseline fetchedBaseline;

if (fetchResult.getRetrievedItems().size() == 1) {

   fetchedBaseline = fetchResult.getRetrievedItems().get(0);

}