Blockstream Green allows you to manage the same Bitcoin or Liquid wallet on multiple devices, with each device synchronized with the latest wallet activity.
This feature is particularly useful if you want to access your wallet on both a mobile and a desktop device.
There is no limit to the number of mobile and desktop devices you can access your wallet from, however each will use a separate PIN (unless you intentionally set them to be the same).
To use your wallet on an additional device, simply restore your wallet on that device using your recovery phrase backup.