Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
If both are good partners, use "both Urshifu formes". It just can't be Urshifu with no modifier. Also, I saw you corrected a previous correction to Urshifu-S to just Urshifu- use Urshifu-S, same principle.