Hi,
I suppose you are referring to cosimulation between the Circuit and PlanarEM in Ansoft Designer.
From the project manager right click the PlanarEM design and choose "Copy".
Double-click the circuit design (the schematic opens).
Right-click the schematic and choose "Paste"
If you are referring to co-simulation between the Circuit and HFSS in Ansoft Designer, from the menu insert an HFSS Design.
Unlike exporting S-Parameter files co-simulation allows you:
1) Automatic update of S-Parameter data when the EM geometry changes.
2) Use EM based parameters (such as width) in circuit level optimization and parameterc studies.
Itai