After Slicing in DM, you should select all the solids that used to be a single part in the Outline and right click then select Form New Part. That will create a multi-body Part that will have Shared Topology automatically applied and the mesher will put shared nodes on the shared surfaces. You may need to do more slicing, such as the outer faces of the tube to get the wall of the tube to share faces with the flange. I have found slicing is faster in SpaceClaim where it is called Split Body, because you can use faces and not planes. In SpaceClaim, you then put all the solid parts in a single New Component, then Open that Component in its own window. On the Workbench tab, click the Share button to create Shared Topology.