Great question! At the moment, each SIM card is a separate Circles.Life subscription. We do not currently have the option to share a plan via multiple SIM cards, although we understand why you'd want this feature. With our 20 GB for $20 Data Plus option, it's just too tempting to pop your SIM into a tablet or other device to share the data. We are taking this feedback in mind and will look into options for the future!
The 20 GB for $20 Data Plus option is only available on the $28 /mo Base Plan.