Hi @jakobtPG23B,
I hope you're doing well. You technically don't need to do this, since you can assign multiple pads to a single pin. So in the device editor you can assign both VSS pads to the single VSS pin.
If for for some reason you still want to do this, you can by appending an @ symbol at the end of the pin name. So it would be VSS@1 and VSS@2. When you look at the symbol on the schematic the @ part gets supressed so it just looks like two VSS pins. This is the old school way of doing it. We recommend the new way.
Best Regards,

Jorge Garcia
Product Support Specialist for Fusion 360 and EAGLE
Kudos are much appreciated if the information I have shared is helpful to you and/or others.
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