The GigSky app has built-in technology to recognize the device being used. The app for dual SIM iPhones will only allow you to manage the service for the phone being used.
For legacy iPads that do not use the Apple SIM and use a standard GigSky SIM, you will be able to manage all SIMs from the web app at app.gigsky.com. For iPads with either the physical Apple SIM or embedded Apple SIM, you have to go to Settings and add a data plan from there without the need for an app or the web app.
You can log into your GigSky account at app.gigsky.com from any device and manage all your GigSky physical SIMs.