You can access a single save data file between multiple devices.
On the device you wish to share the save data with, select Link a Nintendo Account when you first launch the game, then link the Nintendo Account with the save data that you wish to use.
Notes:
・ The same Mario Kart Tour save data can't be accessed on multiple devices at the same time.
・ Rubies that have been acquired or purchased can only be used on the same OS. They will not be usable on devices that use a different OS.